In my opinion, the longer you keep your pullets separated from your flock, the harder the integration will be. I have raised several clutches of new chicks, most raised by mother hen. The hens will introduce the chicks to the flock between 3 and six weeks. Although she will protect them for a time, they usually abandon them when they start laying the next clutch. I have never had a problem with any chicks killed or injured from other hens especially the roosters (I have 6 roos) One observation of note, while mother hen is raising her chicks, she does not appreciate other hens coming to call. The roosters though are never discouraged from checking on the chicks. I believe that this is important so the alpha roo knows these chicks are part of his flock. Just my belief.
I have experienced that clutches will often form sub-flocks until they are nearly grown, especially if there is a cockerel among them. The siblings will hang together, away from the older birds, during the day but will return to the coop to roost. I have even had some clutches, with dominant cockerels, that established a separate roosting area. I discourage the separate roosting area to avoid future fighting and to keep maintenance to a minimum.
My birds are free ranging.