Ok, for a small tank:Bolivian rams, German rams and other color morphs, Laetecara curviceps or dorsigera, any apisto.
55g and up: Geophagus, firemouths. From what I hear, oscars aren't too aggresive, they just get 14 inches long and eat what can fit in their mouths. (They need a 75 gallon minimum too.)