You've got plenty of options with you current setup for more tank mates. I'd avoid anything that's truly aggressive; Vieja's, jags, blackbelts, anything with the name terror or devil, etc. Focus more on semi peaceful medium sized ca/sa like Severum's, Firemouth, Nicaraguan, Acara, etc.
Like with all cichlid setups, I'm sure there will be some trial and error, as every fish is different.
I'd also suggest a group of dithers/targets to not only difuse agression but increase in-tank movement; Silver dollar's would do great with that grouping but aren't your only option.
Good luck on the hunt and let us know what you end up with!