I have a similar problem. Because my Bator is always on the go during warm months, I always have flocks of different ages. I read in BYC to get the chickens used to each other by keeping them separated but where they can see each other. Then after about a week or two, you put them together at night and by the next day they are supposed to get along.
That has never worked for me so far but maybe I am doing something wrong. The younger flock always feared the older birds. They stuck together during the day and at night refused to go into the coop, eventhough I had them shut in there for two weeks inside a separate mesh cage. I would find them huddled together near the coop but afraid to enter.
I finally gave up after carrying them into the coop night after night and I made them their own little house out of four pallets. When it gets cold though, they will have to go into the big coop because it is heated. By then they will be mature and should all be the same size. Perhaps they won't be intimidated as much by then and I will try again to mix them in the manner that BYC suggested.
