I second the neon suggestion, five seem to do pretty well in a ten gallon. If you want to breed millions of guppies, keep them in a tank by themselves. Otherwise, only keep males or keep other fish in there for population control. Some dwarf cories should do well with the guppies (though they'll probably eat some babies, most fish will) keep them in groups as well, about five. Three or so otos would also be a good addition. A dwarf gourami might also do well, though personality varies from fish to fish. Some small shrimps like amanos or cherry shrimp wouldn't bother your guppies one bit if you just want to focus on the guppies. Platys and other livebearers should also leave your guppies alone. I wouldn't add all of these things to the tank, just giving you some ideas of what you could potentially keep. Do a bit of research on whatever you want to keep, most fish either love to be kept in groups or love to be kept alone.
You say your tank is new, hopefully it's cycled? I wouldn't add anything else until the tank is perfectly stable, so if you're cycling it right now with the guppies, hold off on adding anything else.