I would go with a pair of Salvini or maybe Caquetaia Spectabilis if I were you. Both species only grow to 8-10 inches so I think you'd have enough space for a pair.
well depends, i have 1 indivudual specimen at 13", but that was really rare, other than that one i have seen a couple 11"'s and a few 10"s, i think you'd be able to get away with a pair of saums in that tank, plenty of length room, and plenty of width room.