A monogomous pair of Caudo's can squeeze into that tank. Just keep in mind that while they will often use larger shells for spawning they aren't shell dwellers. They'll also use rocks or other caves, and providing a couple of caves is a good idea.
Shelldwellers need a tank that is wide since they live on the bottom. You could try maybe a trio (1 m 2 f) of multies and see how it goes but, keep in mind you might have to return one female if aggression gets to high.