I'm raising out a group of H. multispinosa right now and they're great. At 1.5" they show a nice rusty gold color, have lots of personality, are active, and get along with each other. A pair of those would be nice.
If you do the South American route, you could get four rams and some tetras, or some Apistos. You may even be able to do a pair of the dwarf pikes.
The footprint of the tank is probably the biggest problem. Gallon-wise there's enough water for a couple pairs of stuff, but given the limited footprint, aggression could become a problem if you end up with a pair of something. There won't be much room for their tankmates to escape the chasing and nipping.