That's pretty much how I was anticipating the heterospila to be, not troublesome but hold their own. Thanks for the feedback!
Talked myself out of the Thorichthys species. Out of curiosity, I checked the only other online source that reliably has American cichlids in, and his stock list has white HRPs available which peaked my interest greatly. White HRPs would look fantastic over my black substrate and really pop. I've never kept them before though, but have always heard they are not nearly as aggressive as their convict (nigrofasciata) cousins. Would the white HRP be a viable option in the 8ft tank with the heterospila?
I'm almost tempted to get them and try it, and worse case devote a smaller tank to a pair once formed if they become tyrannical in the big tank...
I checked the Wet Spot list, and beside Petenia splendida, they have a number of Thorichthys species, all low level on the aggression side.