Pseudotropheus Saulosi, Neolamprologus Leleupi, Altolamprologus Calves, Neolamprologus Brichardi (will outbreed convicts, good luck keeping any tankmate with them), Paracyprochromis Nigripinnis, Really any small tang cichlids,
if you don't care about the biotope thing, you can stick mollies, platies, or swordtails in there. They'll be able to deal with the shellies and they are far cheaper abs easier to get than Tanganyika killies.You have a lot of great options. I think the kribensis idea is a pretty good one. They are usually recommended for softer water but are widely adaptable. My favorite part of that idea is it leaves you room for other fish if he gets bored. You can have a school of dithers to keep activity in the tank or if you plant it heavy enough you can go with small loaches or even a small spiny eel. I also like the idea of a pair of lemon firemouths. A pair of jewel cichlids could work well.
The Tanganyika plan is a good one too. I love my brevis in my 29. Sometimes I do think about getting Lamprichthys tanganicanus to fill out the top level. Another way to go is a pair of julidochromis. I personally enjoy my buffalohead cichilds. They aren't beautiful but they sure are fun. A pair in there would do well and you could have a school of congos above them. If you want color some of the small haps like the ruby green could work.
I know this is a cichlid request but as a kid my 29 mudskipper tank was my favorite tank for years. You can't beat them for personality and antics.