Personally I consider a bantam cock with standard hens an excellent situation. You get the same level of protection and leadership as with a standard cock, while using half as much feed. Plus, most bantam cockerels are ridiculously cute.
How well he will be able to mate will depend on his exact size. A small banty, like an OEGB or Serama, wouldn't, but a larger one like a Silkie or Cochin Bantam probably could. With that said, why don't you want them to mate? A larger bantam would be better than a particularly small one, as I'd be concerned about such as small bird being inadvertently injured by the hens.
And remember that a rooster who genuinely protects his hens is a rarity, standard OR bantam. The most any cockerel will do is call out a warning and keep watch. Very few will actually go after predators. Hence standard and bantam cockerels being equal in this respect.
A suggestion: Get a cockerel who was produced by a private exhibition breeder. There are TONS of breeder quality bantam cockerels looking for homes, available at very cheap prices, and you'll be so much more pleased than with a hatchery quality. They'll be ten times as fluffy, cute, and sweet as your typical hatchery stock cockerel.
