With a tank that size I would personally go species only and get a group of shell dwellers to avoid any nasty conflicts
Edit:
I just thought of other options, you could go for a pair of dwarf acara from SA, maybe these, they like pH =< 7.5: http/fishprofiles....nacara_anomala/
Also Blue Apistogramma seem to handle harder more alkaline water (only checked this one site though): http/fishprofiles....ma_trifasciata/