It pretty much depends on how your other girls treat the babies. I can usually incorporate mine when the babies are about 1/2 grown (sometimes sooner). I put them in a coop that adjoins the big girls' coop as soon as they are able to be outside (3 weeks or so). That way the big girls and boys get used to them. I also let them free-range a little bit together while I am outside with them so them can start with the pecking order thing. By the time the little ones are 1/2 grown, everything is usually worked out and there are no problems.
The other think I have done is section off a small portion of the big coop for the little ones (with chicken wire). I just put a dog house, a roost, and feeder/waterer in there (also a heat lamp). Then, when it's time, I let them loose with the big chickens.
I know everybody does it a little differently, so you'll probably get all kinds of advice - lots of experienced people on here. . .