Right now I have two male guppies in a 5.5 gallon tank who are doing very well for the most part. They sometimes get a little rowdy, but no nipping or bullying. They're active and colorful. Once this tank is cycled I'm going to be adding some RCS as well.
I wouldn't recommend more than two males though. They'll need frequent water changes during the cycling process, and live plants always help!
Don't get tetras at all. They do best in groups of 4-5 and need a lot of swim room, neither of which a 5 gallon tank can provide. If you go for a betta make sure you check him up and down for fin rot (pet stores keep them in those tiny jars).