Once you've made that move, you could probably have a pair of South Americans in the 33, along with a few other fish. They tend to be less aggressive than the others--even than the dwarves--but they require much more maintenance. They are prone to ich, so quarantining is a must, and are sensitive to ammonia and nitrites, as are all puffers. They also require a constant diet of crunchy foods--every meal needs to include snails or other thick shelled critters.
I have both SA and dwarf, and prefer the dwarves. They have more personality and are lower maintenance. I'd urge you to have the tank well planted, as well--they do better with lots of plants, IMO.