Definately pass on the GSP's, they will need saltwater as grown adults, get too big for your tank, and will kill everything you try to put in there with them. If you get two, eventually you will have one.
The figure eights are a little less agressive as a species, but are still more agressive than any cichlid I've ever seen. One per tank, and a 20 gallon is not enough. They can handle freshwater better than the gsp's and are really a brackish fish. I kept mine for years in freshwater. They are pretty good little puffers, but you must feed them snails to keep their teeth from growing to big.
If your serious about keeping puffers, you will need to get a bigger tank, or go with the peruvian puffer or dwarf puffers. Both are totally freshwater, and readily available. I've even seen them at walmart, or your lfs can probably order them for you. Bothe the GSP and the F8 will outgrow your 20, as both get to 6"-8" or so. Most don't in captivity because they are not given room, and not given a good diet or water conditions. Remember, puffers are messy fish, good filtration is essential, and room to grow. They will grow fairly slowly, but they definately need more room.
