Different strokes for different folks, lots of people here like to put small fish in big tanks. I think the only suggestion so far that wouldn't be feasible would be the pair of Jags (or any jags at all). A solo oscar in a 75 gallon would be fine in my opinion. Pretty much anything that tops out at 12" should be okay. Just stay on top of your maintenance. Amphilophus grow quickly and have the "wet pet" personality. If it were my tank, I would probably get an Istlanum, but they don't grow very quickly