Well, as you've found out, chickens are very territorial, so you can't just throw them together. Is your coop big enough so you can add the younger chickens to it, separating them from the older ones for a few weeks?
This way, they can see eachother, and get used to looking at eachother, but not hurt eachother. Then, if you can, let them all free range together during the day, and then keep them separated in the coop at night for another week, and then merge them together.
It can become violent when you integrate new birds to the flock, that's for sure. I try and do it slowly.
Hope this helps,
Sharon
