I like going with a variety personally. Get a bichir for a bottom swimmer, 2 mid sized cichlids (firemouth or green terror would be sweet), and a needle nose gar to swim the top.
Just occurred to me... scats are awesome too if you wanna put a little salt in the water for them. I've been planning on gettting a pair of greens for a while now. You can't beat their mean look when they stick that dorsal fin up.