from my experience, which is limited, you could prob keep a pair of regani in a 29 g. the tank I used as quarantine my group in is just under 20g and I ended up keeping a pair in there for a while. at first they were cool but eventually the female (who was smaller) was hiding all the time and I only saw the male, and that was with quite a few hiding spots. in a bigger tank they are much much more chill. I recently picked up some apistos too and I've found some of there behavior to be similar. you could always try them out for the smaller tank for now