I think as long as they get to see each other it’s easier than just throwing them together. They have time to know the babies are there and they get to familiarize them selves.
I think the age really depends on when you want to add them. The advice I have seen the most is to do a see but no touch with the babies and older chickens. Let the older ones be able to see them but not be able to touch/hurt them. I have read do this for a week try while supervising if the...