Female bettas may go after the guppies too, it depends on their personality. I've had better luck keeping male bettas with my guppies than females, honestly. The females tend to seem fine and then snap one day and start chomping guppy tails, while the males usually show you straight off whether they'll tolerate them or not. I never had a male betta suddenly snap on my guppies.
That said, if you go the betta route, you'll need to introduce them carefully. Add the betta last, and make sure to float it in a breeder net or box for a few days. This lets it watch the guppies without being able to attack, which helps ensure it doesn't mistake them for another betta. Once it can calmly watch the guppies swim by without throwing a fuss, release it, and watch it like a hawk to see if it swaps behavior now that it can actually reach the guppies. If any problems arise, have a backup plan of what to do with the betta.