Different people have different rules. And they all swear its the only way. Personally I add my young (Weather permitting) to the flock at 5-6 weeks. Although I used playpen type method with a rabbit cage. After 2-3 days I let them out and wait around for 10-20 mins make sure nobody is extra bullyish.
I only keep good tempered birds though, no matter what. My roosters don't give them two glances until they get a lot closer to maturity. Occasionally a hen will get extra interested. but they seem to let them be just fine. Mabe they know its the stew pot if they don't get along.