I had a thread about this the other day. Most people said to wait until they were feathered out. I'm going to have some hatching next weekend and plan to try to put them in with my week+ olds. I'll just watch them for a bit, and if there's and funny business, the older chicks are going to sit in the corner.
my hubby wont build me a coop. heck he cant put furniture together, but he can lead his soldiers in Kosovo without loosing one go figure.
I will keep mine together since they are already in the bator together. but i will keep an eye on them, just in case.