When I have chicks to integrate into the flock, what I have done is to pen the chicks outside in a wire dog crate, and when the older birds are ranging they get the chance to see the newcomers(and steal some of their food through the wire
). Then when the chicks are older, I have put them in the pen with the pop door into the coop closed, and left the coop door open, so chicks can't get loose or into the coop, and older birds have access to the nest boxes but not to chicks. Eventually I let them have access to each other, but some fighting is inevitable as they sort out their new pecking order. You just need to watch and make sure nobody really gets hurt, and make sure older birds aren't keeping younger birds away from food and water.
