Should I keep more than one Oto (otocinclus) with my Betta?

Otos are technically schooling fish, and while some people do keep them alone or in pairs, they do best in groups of 6+ in larger planted tanks. I've never had any make it more than a few weeks for me when I would buy 2 or 3.. I never saw them eating.

Then my LFS had a few hundred in stock and didn't lose any during their quarantine, so I bought a ton of then and now I have a bunch (15) in my 55g and they are doing great - nice full tummies and as far as I know I haven't lost any (they are hard to count because they blend in pretty well to any shadowy areas). They do swim together and seem to be much happier (and healthier because they're constantly eating) than when I only had 2 or 3.

From tropicalfishkeeping (parent site) fish profiles:

Compatibility/Temperament: Very peaceful; can be housed in any community aquarium of non-aggressive fish. Must be kept in a group of at least three, and the aquarium should be planted.

I agree with the others about the Otos.
About snails, not all snails reproduce like crazy. Nerite snails are not hermaphrodites but they only breed in brackish water. They will probably lay eggs in your tank but they won't hatch. Mystery/Apple snails also need a partner of the opposite sex to breed but their eggs can hatch in fresh water. The only snails that can really overrun your tank are pond snails/rabbit snails/trapdoor snails (which I think are different names for the same snail), ramshorn snails, and malaysian trumpet snails, to name a few of the common ones. They're hermaphroditinc so they can lay eggs forever and they will hatch. But if you control the amount of food they have available they won't lay as many eggs.

Tank must be planted for these guys IMO. They are very sensitive and always do better in planted tanks. Groups of 3+ is recommend. You can do a group of 3, but I would only do it if your tank was planted.

I wouldn't try it in a 5 gal, personally. Otos are generally pretty fragile as they are wild-caught with cyanide in a lot of places. I wouldn't risk it in less than a ten unless your tank is heavily planted (preferably an NPT) and well established.

Actually, even then I probably wouldn't try it.