I have a standard Barred Rock, bantam Black Silkie, and bantam Light Brahma that are about 5 weeks old. I was recently given a Silver Sebright pair, and the cockerel immediately tries to attack the others when I try to put them together. I have to keep all of my chickens in one pen, though. Is there any possibility that the Sebright will get over his aggressiveness? If not, I have to find the Sebright pair a new home
You have to introduce them gradually. If they all must go in the same run that's o.k. but put the new ones in a cage or dog kennel or something in the run where the others can see them and they can get to know one another. Then after 2-3 weeks try introducing them slowly by letting them out to interact while you're out there. They'll eventually all get along.
I had the same problem, I put mentholatum on the spots where they were being attacked and the attacker got a mouthful of that and didn't attack anymore, they still have tiffs but nothing violent like before.