Hmmm 20 Square Feet of space. well since you are asking what we think...
Personally if I was doing what you are considering (And I might because I like any ideas that say Monster AND Piranha) I would start out with about 36
Pygocentrus nattereri Fry. and as they grow to adult, pull out any that have been damaged until I had a population of about 12. Might have to sell off a few, or pull out some breeding pairs if they form.
Raise them on Frozen, Pellet, Flake, and graduate up to prepared fillets. I wouldn't feed them live, ever. Once the matured fish were sizable enough, I would add in a very large school of young Tetras, such as Colombian tetras, Von Rio tetras, or Dwarf Red Eye Tetras. Something like 100 in the group.
I would do a sand bottom with an earth tone. Maybe some 3M Grade S Black, Brown, Tan, or Red. Plant the tank Heavily with Brazilian Elodea (Egeria densa). Some rooted, and some floating. Maybe a couple other floating plants, and some Crypts rooted on the bottom.
I'd light it with two Metal Halaids in very large reflectors. The plants chosen would be low light, but I would want the shimmer of the the light against the sufrace turbulence to show on the sandbed. Lots of driftwood with moss.
I just Watched the Planet Earth DVD a few days ago which documented Piranhas in the Wild. Stunning video captured.
Some videos you should watch are:
http://www.mefeedia.com/entry/piranha-feeding-frenzy-bbc-animals/14764679
http://www.mefeedia.com/entry/bathing-with-piranhas/16207123
http://www.mefeedia.com/entry/piranha-devours-a-duck/16207121
http://www.youtube.com/watch?v=2sBWBYN5jvA&feature=player_embedded