I've had absolutely no problem integrating six week old chicks. I've even put them in the coop at night and by morning they were all fine together. I have particularly docile birds though. The roos don't bother the chicks at all. Some of the hens do, but by six weeks they know how to stick up for themselves. I do make sure that they have little hiding places that they can escape to if they are getting harassed too much.
My chickens go free range about 50% of the time too, so that may be a factor. Since they aren't all penned up all the time and they have plenty of space to roam perhaps that makes them less likely to pick on each other?