[thespiff]

I don't see why you couldn't add 2-3 Otocinclus to a tank that size. They should do just fine in a well-established planted tank, and are fun little fish to watch. I can't imagine anything else fitting in there though. Definitely not a Pleco of any variety. Even the smallest Pleco would probably cause you grief because they would still be too large and they are all poop factories.