I have kept a male fighter with guppies and not had an issue with either. But having said that others have had problems, i think it purely depends on the fighter and his personality
Make sure you have a back up tank in case it doesn't go well.
The betta may attack the guppies or the guppies may attack him.
Or it could all be good. Keep a close watch on the tank.
Must be careful. My betta, Drago, was a very calm fish. i put a female guppy in his tank, and he swam off and couldn't care less. When I showed my brothers betta a guppy, he went crazy. It all just comes down to personality. The guppies also must have peaceful personalities, as they may nip the bettas fins. All depending on personalities.