a male betta shouldn't be kept with guppies. The elegant, colourful tails of the guppies would make the male betta think he has competition in the tank. He would attack the guppies for this reason.
So female guppies (short little tails and not utterly colorful as much as the boys) wouldn't get along either? I was considering a guppy.. but I suppose I can just set up a 5 gallon with a few and cory cats with no betta