I have 4 wk. old chicks the combs and wattles are starting to show, I would start to separate. Are you planning on keeping the pullets for eggs. Start by pulling the pullets if you make a mistake you can pull the wrong ones at butcher time. But my feelings are you need more space, everyone has given you good things to try. I had a pullet who did this, I would put her in a pen by herself for days and try putting her back and she would go back to it, I finally put her in with the laying hens and they put her in her place. ( she was much to young to be with them but I knew they wouldn't put up with it, I had tryed everything.)Once this behavior starts it is hard to stop. Look them over and the ones who have most of their feathers are the ones who are your trouble, try putting them together.