You have a lot of peaceful fish in there. You would do best with a gentle community cichlid. Yours are also mostly South American species. A green terror would be several steps up on the aggression scale. So would even an EBJD, which is a Central American cichlid. An electric blue Acara would add a nice pop of color to your tank. They are usually pretty laid back and they stay manageable at around 4 inches.
Eartheaters are fabulous but you are looking for a single fish. Geophagus abalios and G. sp. "Red Head Tapajos" are laid back and very pretty even though it takes a couple of years for them to come into their full glory. However they do best in schools of at least 6 and preferably 8. The more you have the more gregarious they are. At first I tried keeping fewer and they hid all the time until I wised up and got them enough friends. They require sandy substrate and they keep it sparkling clean. They do uproot plants as they "graze" for goodies in the sand though.
Wet pets are typically large, aggressive cichlids that don't play well with others. They are kept by themselves and you become their sole companion, thus the "wet pet" designation. Because they have no other buddies they tend to be very interactive and eager for attention from you.
We have a couple of wet pets in solo tanks. One is a red devil and the other is a red Texas, big aggressive Amphoplilus / Amph hybrid species respectively who would happily shred any fish we dared to put in with them. They are great with people. They enjoy coming up to the glass to visit with us and playing with toys that we put in for them (though they still bite hands... hard). They're almost like aquatic dogs. They're also the exact opposite of what you are looking for.
If you don't want a gang of eartheaters, I would recommend an electric blue Acara. Just one, so you don't have to worry about pair drama or conspecific fighting. They are smart and interactive. They won't hang out with you to the same degree as a wet pet but they also won't murder your other fish.
