That's one of the problems that is more severe when dealing with hatchery birds. Its always an issue, but hatchery birds are bred for egg production and fertility, not personalities. For whaterever reason, it seems like many hatchery bred birds have tendencies for bad temperments, which comes out when introducing new stock. It happens with breeder chickens as well, but from my observations, not to the same extent as most hatchery stock. Most small scale breeders won't put up with bad personalities in their stock. Aggressive chicken tastes good......I can share my own personal experience on this. I have introduced younger birds into an already established flock. I would put them in an attached pen where they could see each other until the younger ones were old/big enough to "possibly" defend themselves if needed.
The first new introductions took about a couple of weeks for things to settle down. They're all fine, now. I've just introduced another four young ones. They're going through the "pecking" order. The (two currently on the low end of the order) are making the biggest fuss, often chasing the young ones around until they run back into the hen house.
I keep watching for injuries or missing feathers, and so far there haven't been any. The younger ones DO stay in the house most of the time. They wander out every once in a while, scratch around until the Buff Orpington (yes, I was surprised she was like this) or the AustraWhite chase them back inside. I keep water inside, and put a small bowl of food in there each morning. Anything more than that, they need to go outside to get.
It's been almost scary to watch the establishing of order, and I've needed to refrain myself from stepping in, but so far, things seem to be working themselves out.
One exception to all of this. I DID need to remove my Rhode Island Reds. They were out right mean to the new ladies. I plan to try to reintroduce them in a month or two, when these latest youngsters are closer to full sized. If they still can't play nice, I may need to keep them separate.
Good luck!
Gradual introduction lessens the impact and severity of integration. My stuff has gone well, but I also put the juvenile birds inot a growout pen for a couple of weeks, then make the creep gate accessable to them so they can go in and out to escape.