They need to be at least 16 weeks old and close to the same size as the hen and roo in a standard flock. BO's are normally gentle and easy going. It is the banty roo I would be concerned about. He won't care about size or chicks age and will try to mount them when the mood strikes him. They need to be large enough and old enough to get away or shake him off if his attns are unwanted.
When you do put them together I would put them in a pen in the coop and leave them separate but with contact so that they can get accustomed to one another. Then put them in together at night when they are roosting. They will wake up the following morning and the transition should be smoother.