Yes, a bantam rooster can breed with a standard hen, but he may find it a challenge to get his part properly lined up with the hens' parts, so you may not have the highest fertility in their eggs. And the chicks that result will probably be mid-sized, which may or may not be a problem for you. Mixed-breed chickens are great, but if you prefer full-sized birds to lay large eggs, don't get a bantam roo for a sire.
Also, while bantam roos will diligently husband hens of any size, I don't think that roosters are really effective as protectors. They're good at noticing danger and giving the warning signal, but most are quick to then run for cover as fast or faster than the hens. Few will stand back and try to fight off a predator, bantam roos would be even less likely to do so.
How many hens do you have? Remember that an ideal ratio is 1 rooster to 10-12 hens. Bantam roos will be less likely to wear out their hens so you may be able to get away with having more.