I actually thought neons would have been nice for this tank - only because I was thinking of a fully planted tank - the low depth would make good for growth with less lighting efforts. He also mentioned viewing from above which would give neons a bright color flash as they swim. Not everyone who says neons does it for the same reasons....
A tank for killifish would be nice too, very colorful and small enough to have a group of them, a planted tank would be nice for them too.
Beyond that, if you went brackish you could do some small puffers. You mentioned predatory fish..
If your thinking africans, there are quite a few smaller species that are colorful too..
You can buy shell mixes at a lfs, even Petsmart has some selection usually. A good saltwater shop may have a better selection too.