And keep in mind that the cockerel probably isn't even intending to be "mean." At this age, he hasn't learned manners, plus he's hormone-ridden. He's just doing what nature intended. But that little pullet chick shouldn't have to deal with it. If they free ranged, it probably wouldn't be a...
I would pen him separately. It sounds as if he's experimenting with early mating behaviors, but obviously that pullet chick is too young for that. Plus she's an outsider, and outsiders tend to be bullied by the others until pecking order is worked out. Adding a single bird is never a good...