biggest iv seen them in captivity is about 1.5 inches so that might be a little too small, a 5 g would do for a pair tho
the H. formosa would do great in well planted nano as long as the babies were kept in check
heres my current set up for the sheepsheads im keeping, along with some mollies i hope to trade. the sheepies love to perch on plant leaves and are always darting through the holes in the stump, very fun to watch