i have one of those tanks (2gal i think)
i have one small goldfish and 5 white cloud's in it and i have come to the conclusion it's way too small for any fish,unfortunatly i have all 4 of my breeding tanks busy at the moment so i cant re-house them but once one is free i'll do it or i may put a small pond in my garden.
But i dont think there is a fish that would live happy in the smaller ones of the range. the smallest tank i will entertain now is 18" and only for betta's or other very slow moving fish.
thats just my opinion,maybe there is an expert who would say i'm wrong.