At one of my LFS there are two Mono Sebae's that are really tiny, maybe quarter size. I realize that they get large, but would they live in a 72 usgallon comfortably or would it be too small eventually?
Monos are Brackish fish (actually prefer full marine conditions as they mature), they do well in groups of 5 or more and like a big area to swim as they are schooling fish. 5 would be fine in a 72 gallon tank as long as they were the sole inhabitants or you planned on getting a larger tank in the future.