any of the puffers that sit/dig (miurus, dragon, hairy)
any of the convict type things excluding nanolutea, kanna, honduran nigrofasciata, and spilurus (rio bagaces/tempisque/lagarto/piedras/cabuya/lago nicaragua "siquia", sajica, cutteri, septemfasciata, myrnae)
panamius panamensis
female herichthys
hypsophrys nicaraguensis
any of the lepomis sunfish excluding bluegill, pumpkinseeds, and redear though they will not color up without a harem of females
aulonocara or any of the other mid sized haps make surprisingly good wetpets as solo males, and unlike the sunfish, they will color up without conspecifics, as being the only fish in the tank means you are the dominant fish, thus they fully color up and stay that way
female parachromis multifasciatus or freidrichsthalii
any cichlasoma, aequidens, or andinoacara
any rubricatochromis
any trichopodus except pearls
edit: didn't see the existing tankmates