I've had Guppies in a pair before... they seemed fine to me (Well of course they didn't tell me personally, but that's just my assumption

). Now that I have several, they only interact with each other... 24/7. I always thought they could be kept by themselves, but I've been wrong before. I doubt they would need to be kept in groups, but since they seem to be social fish from my observation, looks like they may need some company.
Sorry I'm not much help. Just sharing my experience... maybe it's of some use to you. Remember, when all else fails...

(*points to siggy*)
-SkyCrystal