I have an ocelatus ...i don't really know much about the set ups ..but they seem to like the ground to be covered in empty snail shells rather than gravel. Plus all the shelldweller tanks i have seen has had this.However i don't think the care what kind of shell it is,a mix of all sized ones should work.PS. the clown loach lives in a shell..hes a weirdo.
Bren