In my opinion, you should be able to keep a group of small schooling fish in a 10gal easily. If they're schooling fish, they are already used to swimming near each other, right? Multiple semi-aggressive to aggressive fish would be a no-no, but I see no problem with even like 5 tiger barbs and a couple of cories. They all stay small, and they're a schooling species. As for 1 fish for a 10 gallon, you could get an easy-to-care-for fancy goldfish,or maybe a dwarf cichlid. There are a lot of gourami's that stay small enough for 10gal, as well as some ctenopomes. Namely microctenopoma fasciolatum and ansorgii. As with any tank, if you maintain it well (and we all know that smaller tanks are more sensitive) it will be fine.