Only a betta should be in anything less than a 5 gallon. Best bet, get a 10 gallon tank and cycle it. SOME male bettas can be okay with male/female guppies. Others cannot. You are taking a risk by trying a betta with a guppy, as the betta could see the guppy as a small male betta... Which results in finding fish parts strewn in the tank D:
My Spartan was okay with guppies, but I doubt the betta I had, Dusk, would even want to be with anything other than his reflection (even that he hated xD)
Also, some guppies are aggressive. Male guppies need groups of 6 or more to avoid aggression problems (then they just think the betta is an overgrown guppy! or at least mine did...). Females same thing. I would not suggest mixing male and female guppies because then you have met the "million fish" :p We had one guy drop off over 300 guppies because his initial 6 guppies bred worst than rabbits.