Pristella/Gold Pristella, small tetra, male mollies, bloodfins/green fire, specialty bottom feeders, guppies of both sex is fine... can only reference the types I have experience with but none have ever given me a problem.
My guppy combo is one flamboyant male and a selection of females. Most fem gup don't get huge but you will get some superguppies. My betta girls will enforce the no fighting rule brutally on non betta, often they've badly harmed a second male guppy so I don't do more than one anymore.
Male mollies on their own are happy energetic schoolers, females on their own are a little lethargic and shy. My pair move around almost as much as the pair of bloodfin.
Remember to look up shoal and school sizes as well as maximum live length of the fish you consider... You can go with what looks good instead of using what fits inside the gallon rating of your tank if you set up a sump style system with overflow skimmer, etc, you can increase your water and plant space without having to have a larger footprint of tank, this lets you have a higher number of fish "in view". Most overflow skimmers are a wee bit HYOOGE for betta and company, tho. You can rig Tom's surface skimmers into a siphon overflow arrangement with some piping work and you can also use five gallon jugs as "sealed" sumps with some work on them. Depends on what you want. Modest sumps are handy for keeping tank level up and expand the water volume by ten gallons easily.
Last edited by Thunderloon; 02-20-2011 at 12:23 AM.
Reason: sealed sump is like a canister filter for an aquarium, you can plant it and put lights on the side if the flow is low enough.