I have a five gallon tank with a betta (Artemus), and a one gallon tank with a guppy (could never figure out a name- can't remember the species- he's just a small dull little guy). Artemus looks kind of lonely, and he seems to have plenty of space in his tank (there's a couple of fake plants, some gravel, and a little hideyhole sort of ornament).
I was wondering if I could stick my guppy in with Artemus safely. Artemus used to live with a Cory (I didn't know you were supposed to keep them in groups), and they were somewhat tense the first day, but then seemed to ignore each other afterwards. The Cory died, however (not due to Artemus). Artemus seems pretty mild tempered.