You don't say what breeds you have. Some breeds are more aggressive than others. I have 21 birds @ present. They have formed roughly 3 tribes, the Matrons, the young layers & the babies but they all get along.
A couple of things I've found help with integration:
- Do it long & slow, months, not weeks. By the time I do a full integration the older birds have lost virtually all interest in the newbies.
- Choose birds of the same or similar breeds in colouring, height, weight & temperament.
- Know what you want from your flock before you begin. I've kept mixed birds & weird lookers with more standard breeds from the start. None of my birds bat an eyelid when I add yet another frizzle or Jap or Belgian to the mix.
- Accept there will be some squabbling as the new pecking order is worked out.
- I deliberately chose less dominant breeds even though they do not lay as well. It has saved me a lot of heartache.
- Get a number equal to or higher than the original flock. This distributes the pecking so no one bird is discriminated against & evens the playing field for the newbies somewhat.
- Getting a group of different breeds @ the same time helps. They grow up together & are more accepting of differences than trying to do it later on.