BYC is the place to come to to shorten your chicken-keeping learning curve. I advise spending some time each day on all the forums just reading for the simple entertainment it affords, and you'll end up absorbing all kinds of useful information about chicken keeping and chicken behaviors. ** I have been doing this for a while. This threads on this site have helped me with other questions tremendously. I just couldn't find an answer that was applicable to my situation with this, well, situation.

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Your posts indicate an interest in chickens and the willingness to learn, which is good. You have a lot to learn. The excellent advice you've gotten already will get you started.
It's already been pointed out you need more space. Go to the coop and run forum for some excellent ideas and an education on why coops and runs need the kind of space that we're talking about. ** will do **
The replies so far haven't really addressed roosters. They can be mystifying in their Jekyll and Hyde behavior. It's not really complicated, though.
You have discovered how like roosters can be to puppy dogs, the friendliness and charm. Then you've experienced how they can turn on a dime when you bring hens into their lives. **True that**
Roosters that have started out relating to their humans on a peer basis as Elvis did, then will see those same humans as competition and a threat when hens enter the scene. The first lesson to learn about roosters is you can't have both a puppy-dog roo and a flock roo. you can have the first if you don't have hens, but if you have hens, you need to maintain a strict separation between you and the rooster. ** We know this. We aren't looking for a puppy dog rooster. Just one that will hang with his girls and not feel threatened by us like Elvis was. So far so good with Link. **
There are a lot of great tutorials here you can read that will educate you on how to relate to and discipline your rooster. Please take advantage of them as problem roosters can be a constant headache at best and a safety threat at worst.
I want to point out that keeping a rooster does not insure that your flock will be safe from predators. Roosters are every bit as vulnerable as hens. However, many have sacrificed their lives to a predator so that the hens will be spared. That's all you should expect. ** That is all we expect. We are in a high predator area both from land and air. I want to give our girls the best chance at a long life. **
Replacing Elvis with a young cockerel has merely postponed the reckoning you will face when this new boy becomes mature. Cockerels require a good two years to learn their role in the flock and to become adept at keeping the peace among the hens. It's a beautiful thing to watch a good rooster break up a hen fight and know exactly which one has been the trouble-maker. ** Isn't he considered mature (ish) when he is actively mating with the girls? And though he picks on zelda at times in the coop, he will still 'yell' when a bully pecks at zelda when they are free-roaming **
You need to discipline your cockerel starting immediately to know his proper relationship with his humans or you will face the same problems you experienced with Elvis. For me to tell you all the things you need to know in order to do this would require an even lengthier post. Go read those rooster tutorials. ** We do. He respects us. The woman at the rescue showed us how to grab him and hold him by his feet upside down and to football hold him belly up to show dominance. We do this from time to time to keep on top of that **
Finally, you need to understand that chickens to not need "buddies". Trying to introduce ever more new individuals into your flock because you think a pullet needs a friend is misguided. New chickens coming into the flock upsets the social dynamics every single time. It's the last thing you want to do except to purposely increase your flock. Besides, as long as a chicken, no matter how persecuted it is by the flock, has other chickens in proximity, they are content. ** This answers my question on adding another hen perfectly. Thank you! **
One other thing, when you try to protect this picked-on bird by removing them from the flock it only makes it that much harder on them when you return them to the flock. The social order will eventually sort itself out, but all members of the flock need to be present in order for it to do so.** She is only removed when we are not there, and is in a wire cage next to their coop/run (unless it is raining, then we have her in the garage. She free roams with them and roosts with them otherwise. Now that we have the extended outdoor run, we are working on keeping them all together as much as possible, since she has room to get away. **