Have you considered a group of thorichthys?Very interesting to see the direction this thread has taken. It's great to know people are willing to pass their knowledge and passion on. At this point I'm leaning more away from keeping a "wet pet" and thinking more down the line of a smaller breeding pair of maybe the Firemouths or similar size species.
I've actually become fascinated by the idea of keeping a group of one of the smaller Geophagus to the point I've put them right near the top of my list.
Worked well for me in a tank your size.
You could have a group of firemouths, 1 male 5 females would work well