Bunnylady,
I routinely add younger chicks to the mix, I don't have any trouble till they get to be a month or so old. Then the little cockerels begin to spar and pick on others. I can usually just remove the cockerels to a separate cage by then and they get along great as long as it's only the boys but I'm careful to try and move them all together. If I have to cage them all together, I remove the first one's for a while and let the newest settle in, then add the others back in and there doesn't seem to be the same territorial-ism.
I put two teeny chicks in with my three 9 day old and one 14 day old chicks and it was so sweet. I took the older chicks out for an hour or so to play with them in the living room and let the newest chicks get used to the brooder then brought the other 4 back and there was no fuss at all. In fact, when it was bedtime, they have been roosting on a brick I put in the brooder to play on and when the older chick (pretty sure it's a cockerel now) was watching the teeny new babies trying to get up on the brick and they just couldn't because they're so small, he hopped down and slept on the floor with a new chick under each wing. They are so smart and so loving aren't they?
