There a few species that would work in a 12 gallon tank. In my opinion, multies, brevis, and ornatipinnis should work.
N. Multifasciatus will live in colonies in large shell piles. However, they breed faster than rabbits, lol, and you'll soon be overrun with them. But they have interesting breeding behavior as all of the adults care for the fry. In fact, the fry are often corralled into "nurseries" where they are guarded by one or two adults at all times. The older fry will also guard the younger ones. I have had a 1/2" fry defend an "eyelash" fry from a Julie. The multies won, lol.
N. Brevis usually live in pairs and sometimes harems. They are a pretty laid back shellie and one of my personal favorites. My Brevis have usually tolerated other pairs around even when spawning and usually tolerate their fry for a very long time. My brevis currently reside in a 55 gallon with calvus, bns, and cyps. They school with the calvus and cyps. They don't discriminate, lol.
L. Ornatipinnis are very pretty shellies. In my experience they do well in single pairs but don't really like anyone else around including their older fry.
L. Stappersi and L. Speciosus could also work as long as there are sight breaks and kept in pairs. My stapps have paired off then tried to kill all the others in the tank. My Speciosus are now doing the same. Both of these species are very stunning.
In my personal experience, L. Ocellatus need more space due to the higher levels of aggression and territoriality. In small spaces, sight breaks are not enough to deter them from beating the crud out of someone. At least thats how it worked with mine.
I hope this helps! If you have any specific questions regarding these guys don't hesitate to ask!
