I concur with FishinPole's post all the way through. Would also add that the surface area has an impact on stocking; a 30g hex is tall which means a smaller surface area for the gaseous exchange. I wouldn't add more other than what FP suggests.
A note on the African Butterfly: how large is it now? I've had a pair of these previously, and they were in a tank of their own. When they get mature (at 4 inches), they will usually eat/catch any smallish fish (like neons) that get near the surface. If your neons start disappearing, suspect the Butterfly. And as others have correctly noted, it is not a cichlid. Pantodon buchholzi is the only species in the genus, and the only genus in the family, and is something of a post-prehistoric fish (which is my way of saying its quite unusual, and unique from most other aquarium fish).
A note on the serpae, if they are the true serpae, they can be very troublesome. Some authors say they calm down a bit if kept in very large groups, but not always. If they kept to their own species with their nipping, wouldn't be quite so bad, but they do tend to nip at anything around them. And before someone jumps in to say they have them and they don't, one must remember than a natural instinct in any fish can sometimes be dormant and suddenly come to life for whatever reason. Knowing what they are capable of, I would not mix them in a small group in a (relatively) small tank, with other fish. A group of 25 in a 100g tank probably wouldn't cause much trouble--but again they might.
[I see Kymmie beat me on the butterfly bit before i finished this, so no intention of upstaging. B.]