We recently added a 3rd dog, a whole new dynamic to a 2nd dog. The Original dog, Ricca, the 5 year old GSD, has seen a lot of dogs come and go through rescue work. So I had pretty good idea of what she needed out of a permanent addition. I like to wait 2 years between adding new permanent additions. So when she was 4, we added Logan. Ricca, is dominant, excitable, and her method of play is "Shepherd smack down" where she throws her front feet out, down, at the other dog, does a 360 in the air, and throws the feet again, then takes off running. She's a flirt too. And has issues with females who display any sort of dominance if I'm not there to keep her in check. So I knew I needed a boy, a Shepherd or similar that would play the same, and something mild mannered to put up with her and not make the house chaotic.
Logan came calling as a rescue, perfect time, perfect dog. 12 weeks old, pure bred Shepherd bred by a moron. He had been attacked twice by another Shepherd on the property, had worms, fleas, and ticks so bad he was anemic, and then Parvo on top of it. He survived all that. And quickly warmed his way into Ricca's heart. She's really good about correcting puppies and acting as a surrogate of sorts in pack manners. So between her and I, he had a good puppy-hood and is now socially acceptable, sweet, loyal, respectful.
As far as adjustment, I took Ricca's crate and gave it to him. Bought her another crate, because she wanted her "den" back. I just left the door open and she weaned herself off it by transitioning her sleeping spots to the center of the house on the floor. Better for guarding. LOL Normally I find her sleeping within the same distance of all the doors. I pulled Logan for one on one training, much to Ricca's dismay. So then I had to pull her for one on one time even though she didn't need it, I wanted to be fair. Then we did group training. If your goal is to walk both dogs at the same time, LOT's of leash work needs to happen and good solid heel needs to be in place, as well as group training going well where they both respect commands with the other present. If they have a mind to chase a squirrel and don't care what you think about it, two large dogs on leash will have the strength to drag you. LOL... I never experienced that, but I spent months preparing for calm, respectful, quiet walks together.
You learn things about each dog as you go. For example, Logan needs to be the inside dog on walks. I don't put one on each side of me, they tend to want to have a pulling contest to get next to each other. So Logan is on my inside left, and Ricca is on my outside Left. Don't bother with the double ended leash, if you need to correct someone, you do it to both. Always use individual leashes. If I change who goes where, it gets thrown out of whack and they don't act right.
Then came the puppy. I'm still wondering what the heck I did that for. Haha. She's been great, and she fits in really well. But Ricca loves her to the point of failing on issuing manners. Logan lets her walk all over him, literally. She is sassy, awful, vocal, demanding, greedy, everything the spoiled 3rd child is. I wouldn't be able to put up with her if she were the only dog. But as it is, it is working. She plays with Logan the most, he's king of puppy entertainment. When he finally says enough, she goes to Ricca. Then myself, then my husband, then me again, then Logan again. She has plenty of energy to go around. She gets right into the thick of it during the Shepherd smack down games outside. She learned the art of body slamming, paw smacking, chase, teasing, all of it. Jumps right into the middle of it and takes a beating from the other two, and loves every second of it.
They exhaust each other, and it's awesome. But they haven't bonded so close that I lose control during a pack moment. I do 3 training sessions and then a group one. One will always come to me during a quiet moment for some loving. Another sees it and comes in too. Ricca will even give her chew bones to the puppy, she's never done that before, she's known as the greedy, bossy, my way or the highway dog. But not with Annabelle. They all get along great.
Annabelle will be starting school soon. I haven't been at the training club in some 15 years, but I think Annabelle will benefit from more one on one time and also working around other dogs she can't take such liberties with like she does at home. She is SO spoiled by my older dogs. I'm the only one in this house that doesn't cut her any slack.
The older dog always needs a lot of patience. They will get chewed on, slapped at, yapped at, all things puppies do. Some dogs don't like puppies, they want a dog they can play rough with and be a "real" dog with. Other dogs, prefer a puppy. It just depends. When choosing a new dog, look for the same key qualities the existing dog has. Play style isn't needed, they'll learn to play with each other. One chases, the other runs, switch... it all works out.
If your dog is prone to high levels of excitement, choose one that isn't excitable to that extent to balance out the other. Unless you want two big dogs chomping at the bit to go outside. I like having only one doing 360's by the door and the other calmly thumping his tail in the sitting position. It's much easier to have the crazy one calm down when the other is consistently calm. If both are crazy, it takes twice as long to restore order. Logan is my sanity, the girls are nuts. But they are very quick to see that Logan gets a treat when he's calm and they get nothing until they calm the heck down.
Some rules I have to avoid conflict, which aren't needed in a single dog household. No dogs on the furniture, period. No king of the mountain games, no "who's closer to the human" games, none of that. You eat out of the same bowl, in the same place, every time. No sharing, no switching, no competing, no checking to see what the other has, no dirty looks over the rim of the bowl. I had to monitor meal time for some 3 months to get that all situated. But food aggression is no issue here. No one is allowed to collect the toys and sit on them. Ricca is bad about that. I'll look over and she's got 6 toys between her front paws with the other two dogs sitting near her peeved about it. There are 3 toys out at any time. No one is allowed to have more than 1, and I can take it if I want it and keep all 3 myself if I want. "Drop it", "Bring it", and "leave it" are solid commands here. No going in the puppies crate and stealing her stuff.
People think it's fair to treat the dogs fairly and equal. But that isn't how it works in dog world. You are in charge. Then the next oldest dog, and down from there. Puppies can get entitlement issues if the human shows favoritism to the cute new addition. That's a good way to have the original dog peeing on your bed. Older dog gets the food bowl first. In group training, older dog that sits first gets the treat first.
If you treat the puppy first because "It can't wait".. you'll have a hard time teaching patience and focus. Puppy is not allowed to steal from the first dog to get the treat. Puppy has to look to you, and wait it's turn. It's low dog on the totem pole, last for everything. Seems hopeless initially when the puppy is spazzing out about who has treat crumbs on the floor. Catch it's attention, hold the treat, practice "stay" while treating the others. In a couple of weeks, puppy will learn that the faster it sits and stays calm, the faster the treats are issued out. It will learn to wait 2 seconds.
Puppy will learn patience, respect, order. If you give the puppy it's food bowl first because you know it will race to the other's bowl, you're giving the puppy power. If it really is that excitable over food, put a leash on it. Control it's movements. You'll thank yourself later for the early training and manners when the puppy is a year old. If the puppy gets special treatment because it's crazy and cute, it will challenge the older dog, play too rough, and act like it's high dog when it's 1 or 2 years old, because the human said from day one that it has that power.
But my puppy is crazy. She needs more care and attention and training than any puppy I have ever owned. Logan was a very easy addition. But since you over analyze, the worse that can happen is you end up with a lunatic puppy that requires twice as much training time as you've put into your current dog. Or that their energy levels feed each others excitement and your time will be spent keeping order.
Just be patient, choose wisely, let the dogs meet and great before making a decision. Don't rush it. Don't show total equality or favoritism when you bring a 2nd one home. Just keep the natural order of things. Don't slack on training or exercise because they have each other. Do exactly what you're doing now x's 2. Training will be x's 3, one on one with each, then group. Have your rules in place and hold to them. If the current dog is allowed on the couch, will the 2nd? If not, start teaching the current dog no more couch, before the new one arrives.
It's fun. I'm excited about this Spring, when all my training pays off and I can walk 300 pounds worth of dog by myself. We've been practicing in the yard, with the puppy on my right.
. She is well trained, but is constantly pushing to see what she could get away with. Then about 14 months ago we got another because Sadie just couldn't hunt like she used too. Jethro is the new 14 month old male weighing in at 105lbs. He is also well trained and a pleasure to be around. After we bought the new puppy home we had problems with the female tearing things up around "which had never done her entire life" the house and picking on the puppy. I guess she was trying to make a point to him that this was her house and she was the queen and she didn't have to answer to anybody. Including the wife and I. When ever one got something the other also got the same. Now that Jethro the male has grown up she has stopped all the nonsense and acting normal again, but there always seems to be some tension. They both have been kept in the house their entire lives and we spoil them rotten. Neither have ever spent a night in a cage or locked up. 
