I have one gsp in a 5 gal for now, working him up to full salt.
Gunna have a 50 or 65 gal for him soon...also going to scats, monos or archers as tank mates as I have heard of success with them before.
Here he is at feeding time X) http://www.youtube.com/watch?v=vmMi7QvE1XI
Alyssa, I'd get that puffer into at least a 10-15g ASAP! Scats are huge garbage dumps & need a minimum of 50g ea. Monos are schooling fish & need the same water volume per fish as the scat. 1 archer may work, as long as the tank is wide enough for it. Please look into the adult sizes of the fish you are looking at & their recommended tank sizes. If I were you, I'd let the GSP be the centerpiece & get a nice school of colorful damselfish, when he reaches marine conditions.