My bettas do okay with mollies, mickey mouse platys and the like, but your tank isn't large enough for anything but maybe some small shrimp (I have ghost shrimp help keep their tanks clean). It also depends on personality, too. My betta, Jewel, will eat any shrimp with her, but the rest of my bettas do fine with their ghostly partners.
If you want anything, all I really suggest for that tank is 1-3 ghost shrimp. They're interesting little critters and helpful to boot. You'll just have to buy some algea wafers and feed them every other day (sometimes every 3 days if they don't eat it all) and be careful about adding them to newer tanks. There's no bacteria or algea buildup in new tanks, and they may not survive.