I'd say the only fish that would fit in that would be either a betta or some of the micro fish. Male guppies should really be kept in larger groups to stop one fish being bullied and one becoming the boss. They also don't like to be alone.
When you're talking tank size you need to think of the amount of fish that the tank can support and also the size tank the fish need. Like, I could put a six inch cichlid in a five gallon tank. The tank's capacity to support fish would be adequate to keep it alive in terms of coping with its waste and respiration. But the fish would be desperately unhappy in such a tiny tank.
Bettas don't mind small tanks. I have kept them in smaller tanks than 2 gallons (1.5 is the smallest) out of necessity. I would prefer to use larger tanks but I don't have the option. I just try really hard to provide them with interesting, stimulating environments. I would rather live in a planted, decorated 3 gallon tank than a bare 6 gallon.