go to liveaquaria.com and look under their fish section, and then go to the nano fish. I think that most of those are good in 12 gallons, but always ask here first before you get them cuz liveaquaria isnt always right.
But i'd say gobies would be fine. Like yellow watchmans, clown gobies, thers really too many to list.
Inverts...i'd get about 3 cerith snails, maybe a few astrea snails, and then maybe a cleaner shrimp, but they dont really clean up the tank...more the fish.