I have a 2.5 gallon and wouldn't dream of putting anything in with Guppy... He tends to be very aggressive so I don't think he would be happy with a community, no matter how big the tank. He would freak out. But his sister Bubbles has a completely different personality, she is a much smaller fish and very curious. She actually likes change and I think she would do well with company, but not in the small tank. That is is an only fish size. I know some people keep companion fish in a 5 gallon and they are just fine. They do have live plants though so I suppose that helps with the bio load.
My yellow HMs King Willem-Alexander, Queen Maxima, Rosie, Pepper ...and ten thousand children of Willem and Rosie (it is really about 80, but it seems like ten thousand).
RIP Guppy... The little fishie who inspired me to breed bettas.