You are dealing with living animals. Anything can happen. A lot of us integrate chickens all the time and it is usually no big deal, but occasionally there are serious problems. There are some things you can do to improve your chances of success, but no one can give you absolute guarantees. Each individual chicken has its own personality, each flock has its own dynamics, and each of us have different set-ups.
There are two kinds of aggression you have to be concerned with, assuming you do not have two roosters. Roosters add a third one for flock dominance. One type of aggression is purely territorial. Chickens are flock animals and accept that other members of the flock have a right to exist, but they will sometimes attack strange chickens. Sometimes, not always. Sometimes this is not a problem at all. But you can help this not be a problem by housing the newcomers where the older chickens can see them but not get at them to attack. After maybe a week, they should accept the right of the newcomers to exist. This does not always work. Each chicken has its own personality. But it really improves your chances.
The other aggression is the pecking order. For the flock to get along peacefully, each chicken needs to know where it ranks. That is usually determined by intimidation and usually involves some pecking. What usually happens is that the dominant chicken pecks a less dominant chicken that invades its private space. If the non-dominant chicken runs away, things have been settled. If the chicken does not run away, a fight ensues because that is a challenge to the pecking order. Sometimes the dominant chicken will chase the other but as long as the weaker can run away, things usually work out. You do occasionally get a hen that is a real bully and brute that will actively seek out weaker chickens to do real damage. It does not happen a lot to the point that it is real serious, but it occasionally happens and it is almost always a hen.
A mature chicken will always be more dominant than an immature chicken. With yours, your newcomers are almost certainly too immature to establish their final position in the pecking order. I've had some that could do that at 4 months age but that is really rare and is practically always a rooster. Most pullets seem to be able to do that about 6 months, though I've had some that took many months longer. Roosters usually take longer too, but occasionally there are exceptions. Each chicken is an individual and each flock has its own dynamics. Just one chicken can change the whole flock's dynamics.
If the weaker chicken does not have room to run away, it can get real serious. The more room you have when you integrate the better.
So some of the things you can do to help improve your odds of success are to house them side by side for about a week before they can physically get to each other and provide them as much space as possible when you do let them get together. Provide separate eating and drinking stations so they can avoid conflict just trying to eat or drink. Make sure they have plenty of roost space. Mine can be really brutal on the roosts as they are settling down to sleep. Extra roost space gives them a chance to get away. I usually integrate my brooder raised chicks at 8 weeks. They have a separate place to sleep which I think helps. But my broody raised chicks sleep in the big coop with the big chickens even after they are weaned. They really appreciate the extra roost space when Mama leaves them on their own. I move my brooder raised chicks into the main coop at 12 weeks. They like the extra roost space too.
Usually my integrations go so smoothly I wonder what all the fuss and worry was about, but occasionally they get a little rough. I let them sort it out. I’ve never had a chick die because of an integration, but some people have. I really think space is the most important thing you can provide, but each chicken has its own personality and each flock has its own dynamics.
I see others have been responding while I was typing. I don’t worry too much about physical size. Bantams will never be as large as regular chickens. I find it is the spirit and maturity of the chickens that counts a lot more than size. It’s not unusual for mature bantams to dominate mature full-sized chickens because they want it more. I’ve had broodies wean three and a half week old chicks. Those chicks certainly did not have the size or maturity to stand toe to toe with the adults, but they made their way in the flock. With enough room, they simply avoid the older birds.
Good luck!!!
