A lot of us integrate fairly young chicks all the way to adults into our flocks all the time with little or no drama. However, some people have chickens die when they try it. It might help you to understand some of what is going on.
Some chickens are territorial or protective of the flock. Chickens know who belongs to the flock and who doesn’t. Some chickens might attack a stranger. This does not happen all the time, but it does happen. This is why housing chickens side by side for a week or two before you let them together is a good idea. It doesn’t stop all aggression but it can make a big difference. It can also help if you can first put them together in neutral territory (not many of us can do that) or the territory of the weaker ones (I don’t do that either). Sometimes you just have to do the best you can.
Then you have the pecking order. Each chicken has to know where it ranks socially in the flock so it knows whether to defer to another chicken or if that chicken should defer to it. Other social animals like packs of wolves or herds of cattle do the same thing, they have their social ranking too. The way chickens do it, when two chickens that don’t know how they rank meet inside their personal space, one pecks or tries to intimidate the other. If that chicken runs away, it’s settled, although there may be a bit of chasing or repeat lessons. If one doesn’t run away, they normally fight. It usually doesn’t take long at all for one to decide they are better off running that fighting, so it is soon over. It’s only when the two are really evenly matched that this fighting goes on for long. That’s pretty rare. But they are trying to hurt each other. If one becomes injured, say they dislocate a leg or more normally blood is drawn, they can become really vicious. Chickens can become cannibals when they see blood.
Most of the time, there is no fight, one quickly runs away. Even when there is a fight, it’s normally over long before one gets hurt. Even when they see blood not all chickens become cannibals. There is risk in these pecking order fights but usually not a lot.
One huge key to this is that they have to have a way to run away and get away when there is some chasing. If one cannot run away the winner does not know it has won. It keeps attacking, usually going for the head. The loser often hunkers down and tries to protect its head, but it can be murder. You will see all kinds of magic numbers as to how much room per chicken you need. When you are integrating, throw those numbers out of the window, they mean nothing. It’s whether a chicken can get away if it needs to. This means give them as much room as you can. Also, give them other safe places to escape to. If the roosts are high enough so they cannot peck them from the ground, roosts just might be a safe place. Or if you are integrating smaller chickens, give them a safe haven where the adults cannot follow, something like Azygous described.
Immature chickens add another level of complexity. More mature chickens always outrank less mature chickens and some chickens can be brutes and bullies about this. They may go out of their way to attack the weaker chicks. This can be adults versus younger chickens or teenagers versus preteens. Again, this does not happen all the time, from what I’ve seen this level of brutality doesn’t happen all that often. But it does happen often enough to be a serious concern. That’s why room to run and a safe haven are so important. Something that I’ve observed many times. A baby chick around two weeks old being raised by a broody in the flock leaves Mama’s protection and goes to stand next to the other hens at the feeder. Sometimes the other hens ignore that chick but it usually doesn’t take long for a hen to peck that chick to remind it that it is bad chicken etiquette for it to invade the private space of its betters. That chick goes running and squawking back to Mama, who ignores all this. But on the rare occasions where the hen follows that chick to reinforce her message. Mama politely and thoroughly kicks butt. She’ll let other hens teach her chicks manners, they need to learn how to interact with the flock, but nobody hurts her chicks!
When I integrate brooder raised chicks I see sort of the same behavior. I normally integrate these chicks anywhere from 5 to 8 weeks old. On rare occasions I’ll see one of these chicks approach an adult and get a peck, but usually they know they are so socially inferior that they are afraid of the adults. They form their own sub-flock, living on the outskirts of the main flock. They are smart enough to avoid chickens that might hurt them. Again room or a way to avoid is very important. This may be pure room, it may be a safe haven, or possibly a place to stay out of the personal space of the adults. It’s really normal when I open the coop in the morning to see the adults on the floor and the younger chickens on the roosts where they are safe.
Normally pullets are clueless until they start to lay, or somewhere shortly after that. They don’t know what is going on with boys until then, but when they can force their way into the pecking order, they do OK. Boys are a different situation. During adolescence their hormones are generally out of control, they are often big enough to be bullies, and they have no self-control. Pullets run from them, as do some of the older hens. But the more dominant older hens may knock the snot out of them. It’s not always about size but more often personality. Eventually those cockerels mature into roosters but until then things can get really wild down there. If they have enough room for pullets and some of the hens to run from the cockerels or the cockerels to run from a mad dominant hen things normally work out but this phase might be hard for the faint of heart to watch.
I suggest if you are going to add a rooster to your flock, don’t get an adolescent. If you get a chick and raise it with the flock you will probably get some drama during its adolescence but it’s usually not that bad. Or if you get a fully mature rooster, say at least one year old, and you have mature hens, he normally just WOW’s then with his maturity and magnificence and takes over. Usually not much drama there.
A dominant rooster is not likely to be a problem when integrating chicks as long as they are fairly young. He will see them as his offspring and won’t harm them. It’s not that unusual to see a mature dominant rooster help take care of chicks. The hens, adolescents, and non-dominant rooster are more of a threat, though a mature non-dominant rooster usually isn’t too bad.
I let my broody hens raise her chicks with the flock. She takes care of basic integration for me but the chicks still have to go through the pecking order issues as they mature.
My brooder is in the coop. I have a grow-out coop next to the main run that I sometimes use. The chicks grow up with the flock, just behind wire. Sometimes I move them to the grow-out coop until they are 8 weeks old then let them roam with the flock. Sometimes at 5 weeks I just open the brooder door and let them mix as they will. I have an 8’ x 12’ main coop, a 12’ x 32’ main run, plus an area about 45’ x 90’ inside electric netting. They have a lot of room. I feed in three different spots and water in four so there is no competition for food and water.
I’ve probably made this sound horrible and really dangerous. A lot of the times this goes so smoothly that you wonder what all the worry was about, even when you don’t take any or many of these precautions. Some people just put new chickens in the coop at night so they wake up together. Occasionally they wake up to dead chickens trying this but most of the time even this works. Don’t try this with chicks unless they are in a safe haven.
I have never lost a chick to another adult in the flock in all the years I’ve been doing it my way. I know others have, when you deal with living animals you just don’t get guarantees, but it doesn’t always have to end in disaster. I find the keys are introduce them slowly and give them lots of room to run away and avoid.