When the young uns get close to the size of the old uns, approx. 8 weeks, is when ours usually get put together. It DOES help to let them see each other thru a fence/chicken wire for a week before they actually mingle. The more space the better, so the underdogs can get away from the dominant ones. If you keep them confined, 4 sq feet per bird minimum for the coop, plus more space in a run or yard is even better. We let ours free range during the day. There was a little scuffling over roosting spots the first couple nights but not much drama at all. We merged 8 young uns and 10 new mature hens into an established flock of 16 all in the same week. If you get a persistant bully, stew her. If anybody gets bloody, separate them in a dog crate or something until they heal or everybody will want to pick at the sore.
Free ranging chickens are great bug control and they help with parasites because they love to excavate fresh cow patties. I have seen them walk around the cattle when they are bedded down and pick horse flies off of them. And besides, don't you cowboys need some eggs to go with that steak???