It's not recommended to keep bettas and guppies together. Most people will say this is because bettas will mistake guppies, with their flowing tails, for another betta, and attack. And this can definitely happen.
However, as you've found out, guppies can be quite the fin nippers themselves, and not every male betta is aggressive. I have lost bettas to aggressive male guppies in the past, very peaceful males who never so much as flared at anything else in the tank.
You will need to keep them separated. Bettas, in my experience, are far betta living alone or with very peaceful tankmates like corys. It's not fair on either the betta or guppies to have them live together.