Lots of options, just try to get stuff that is slightly different from the others species, thats where problems usually start.
Ie: my old Nicaguan would attack anything to death that was the same shape, but would live peacfully with things like Angels, Severums etc.
You could do...
Blue Acara
Firemouth
Female Convicts or HRP's
Geos
...all sorts really, depends what you want.
If it was my tank... I would do one Rotkeil Severum with a few different smaller types.
Rotkeils are a smaller Heros Sp. with the Largest known specimen being 8" (not including tail)...The Common Severums (Efaciatus...usually labelled as Severus...ie: the Greens, Golds and Red Spotteds) can reach up to 12" (although 8-10" is the norm for males...females smaller)
