for a pair i think a pair of sajicas would be cool, but as for a wet pet i think a single male firemouth would probably be comfortable in there by himself, they have some great colors if you can find a good one
Some dwarf pike like regani would be cool, if you could get your hands on them. Sajica would be pretty awesome too. And one last idea, Laetacara dorsigera. Not quite as aggressive but still a very neat fish.