If you're considering SA species as well, I'd personally avoid any of the really big boys, Umbee/Azuls in a 6' tank. Dovii for centrals. Aggression will vary form fish to fish and pair to pair, but every fish listed in this thread have potential and the smaller the tank the higher the odds of it manifesting.
Festae would be my pick if you don't mind going SA, but they are more aggressive than some. Any of the Vieja/Paratheraps family could be really cool too. Most of the herbivores get pretty big and aren't too brutal during spawning, for the most part, but that can change.