Many of us have found that the sooner young birds are integrated into the flock, the better. Ideally, IMO it should start as soon as the chicks are off heat. Many others do so even sooner. When the pullets are reaching sexual maturity, that is IMO the worst time to integrate, as at that time they are considered to be more of a threat to the established pecking order. When they are still peeps, they get a cursory peck to remind them to mind their p's and q's. But when they are becoming sexually mature (have their clucking voice) the older birds are much more aggressive.
Aart raises an important question. How big is your coop and run? To successfully integrate, you need more room than the generally recommended minimum of 4 s.f. in coop and 10 s.f. in run/bird. Multiple feeding stations helps, as does providing plenty of hiding and multiple level places especially in the run. If you free range at all, that is the perfect time for that first face to face. I like to have plenty of treats that I can scatter around when integrating younger birds. The old biddies can guard the feed bowl, but they can't guard a whole section of lawn that has lots of tasty stuff scattered around. They are too busy grabbing stuff (kind of like at a Pinata party) to worry about guarding a small area. When your birds are free ranging, the youngsters can have plenty of retreat space, and they can also join the periphery of the flock, and work their way in gradually.
As for your cockrel: I have concerns about his temperament as a flock leader. A good roo will guard the youngsters, and he will not be aggressive towards them. In fact, he will likely step in if one of the biddies harasses the chicks. He will never breed the young pullets until they are close to POL. Granted, your cockrel is young yet. And he has not had any one to teach him proper roo manners. You might be able to do a bit with him by putting out the treats, and driving him away from them to allow the youngsters to eat first. Don't let him back to the treats until YOU say he can come back. This is accomplished by having a skinny stick about 4' long. You simply place yourself between him and the treats/youngsters, and any time he approaches, you chase him away. a light tap on the heiney with the stick with you following along, and he'll get the message. Roos are smarter than we give them credit for. When my roo is in pursuit of a low pecking order gal, All I have to do is verbally reprimand him. And he'll change direction, acting like the cat who just fell off the couch. "I meant to be going over here anyways!" Either way, I'd take him out of the picture: temporarily for at least a month, or permanently. Good roos are a dime a dozen, and you don't need one who is terrorizing your pullets.
