Hi Taylorm
smb is pretty much on the money, the Brazillian Colomesus asellus is likely the mildest of all puffers, but is known to fin nip occasionally (especially slow moving/long finned fish). If you are thinking of any cichlids be sure they are non aggressive. I have kept Ottos, guppies, & sword tails with mine for a while now with no real trouble. You could keep a pair in a 20 gal nicely.
As for the GSP's Tetraodon nigroviridis I wouldn't recoment less than a 29 gal for a single, they get up to about 6 inches and are very territorial. Being BW species you are pretty limited if you want to tankmates, maybe some Bumblebee Gobies or Orange Chromides.