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I think there are some new worlds but I can't speak for them so maybe someone else will chime in on that for ya. With Africans you may be able to do a few of the really small mbuna. Lake Tanganyika has shell dwellers that would work imo. Not to many people that I know of keep them so I think that be cool. There are a few shell dwellers from Lake Malawi but finding them in the aquarium hobby probably won't be easy. If you do go the small mbuna route it will probably need to a species only tank. Personally I would go with salt you'll have more things you can do like corals.
There are several different genuses of dwarf south American cichlids that will fit great, and some smaller species as well. Try looking into apistogramma, dicrossus, mikrogeophagus, laetacara, nannacara, keyhole cichlids (cleithocara maronii), bandit cichlids (guianacara sp.), rainbow cichlids, fire mouth (as well as other thoricthys species like eliotti), Cupid cichlids (biodotema cupido), festivum, to name a few
As for the shell dwellers, I personally keep neolamprologus multifasciatus, the smallest known species of cichlid and I love them, but they are super limited to what can be kept with them to basically nothing. I have ghost shrimp that are terrified to death of these little one inch fish haha it's a blast seeing how they dig. Two of mine get into these wars where they almost try to bury each other in sand lol they will completely cover each others shells, back and forth