I have many tanks, but I want him to get another 2" before I throw him in the 55. He is still really young (has external gills) and growing fast, so it's purely an interim solution.
I don't know if a dwarf puffer would do well in a tank that small. It's not even the size I'm concerned about, it's the fact that the filter that comes with it wouldn't be able to handle how messy the puffer is.
Since shrimp seem to be a non-starter...pygmy sunfish! It's a perfect tank for a little colony of them, and they won't tax the filter like a puffer would.