Normaly we keep them totaly seperated until they are full grown pullets ready to lay, then they get thrown in with the others and they have to duke it out for the pecking order. It isn't all that bad as the ones that grew up together stay together until full integration. Most folks will way over think the whole integration thing and freak out to much if some get injured or rejected by the flock, it's how animal husbandry goes, if you can't do this then start building a whole new set-up for the new birds, and then another for the next batch of new birds, see where I am going with this. Perhaps the ladies will chime in with more sympathy, I however prefer to advise on reality and common sense if that's exceptable here.
I grow out chicks in the adjoining pen as the hens. They will have spent 3 months "talking" and watching each other through the netting. Still, when the day of integration comes, it is not a pretty sight. Chicken politics are tough. It takes days, but they work it out. I integrate when the "chicks" are 16 weeks old and can match the physical size of the older hens, not that the older hens care!!! Gosh, they can be nasty.
Lol thanks guys I started by letting my chicks out with supervision for a few hours a day. They have always been caged in he coop with the big girls. Just recently have I been leaving them out most of the day unsupervised and they are just about eight weeks.
My chicks are raised next to adults -- there is fencing separating them. I mix them together at around 14 weeks or so. I generally let them free range together for a few days -- they naturally go to their own sides of the coop at night. Eventually, I just don't make the chicks side available. There is always a pecking order that has to be established, but I've never had any blood drawn. The young ones are always submissive to the older hens anyways, so there's never any trouble. I like to do it while they are free ranging because everybody has more than enough space.