Platy are my absolute favorite tank mates and they have pretty much the same bioload as bettas (from what I've read, I'm not an expert!) so the same "inch of fish per gallon" rule seems to apply to them as far as stocking goes. But you do have to be careful about the gender of the platy and even out the number of boys vs. girls. You should have a higher percentage of girls than boys when you have platy. They also prefer groups of three or more though they are not really considered schooling fish (I don't think so anyway).
Other tank mates I have are Glofish (danios), Glolite tetras (much hardier than neon tetras) and I even have one Clown loach which also are recommended in pairs or groups, but Rockzo's mate died within days of purchase and he's been very happy without. Clown loaches can be tricky because they are VERY good hiders. Literally, you will think that he jumped out of the tank and ran away or something, but he's probably just hiding under the gravel. They find the smallest hiding places they can sometimes, and they are generally nocturnal, although Rockzo is always out and about. My favorite characteristic of the clowns is that they will try to imitate the other fish around them as if trying to blend in. I gotta tell you, it's one of the funniest things to see a bottom dwelling fish, following my alpha betta female, journey to the surface for air, take a gulp, and then scurry back down to the bottom, just like a betta.
Woah, that ended up way longer than I thought it would, haha. So yeah, those are my two cents!