Is he actually ripping out feathers with his beak? Have you seen it? When roosters mate, they balance on the hen's back, holding on to her head feathers for balance sometimes using his beak. The spurs and nails on his feet can tear back feathers to the point of balding and can also rip skin if they get bald.
It could be that he is just too large for the bantams and has to grip so hard that he is pulling out feathers.
It could also be very intentional. We had this happen when we found out our frizzles hen was actually a roo. We have 5 hens. Our older roo was very obviously not pleased with the new rooster, and his method of showing his dominance and displeasure was to abuse the hens, particularly any who interacted with the new rooster. He would chase them until they squatted, not even attempt to mate them, and rip out feathers. Eventually 2 of the hens removed themselves from the yard every day and wouldnt go near him and started laying bad eggs. Fart eggs, soft shell, shellless, corrugated. We culled the older roo and immediately everyone has integrated and calmed.