I tend to keep a pair of dwarf pikes and a pair of dwarf acaras (Laetacara, keyholes) or dwarf pikes/rams in my 40 and 50 breeders (36x18x16 or 36x18x18) tanks. But then I am partial to pikes I think. I was thinking Biotodoma since I love eartheaters, but after seeing adults and how thick/massive they are, I don't believe them to be doable (though I am sure others will disagree .. but I tend to be conservative when it comes to stocking).
Before I've done single pairs of midsized (5-6") south or centrals, like blue acaras, ports, convicts, ect.