I have to agree entirely with BWG on this one, though I'm sure you already knew that. . .
IF you were housing the Puffer in a tank of the proper size, perhaps then we could look at possible stocking options, but I can not in good conscience recommend that you add anything into this tank, only re-home the Puffer, as we spoke about in your previous thread.
Considering all you have now learned about Puffer fish, I must admit that I'm fairly shocked to see you even considering adding any other creatures into this setup. 5.5 gallons is still too small, far too small, for this fish to lead a healthy, happy, or long life. . . even all alone.
We all make mistakes, we all started somewhere, and many of us on the wrong foot. . . we go from there, we fix our errors and set things right, we learn - and we try not to make any more. Please don't do this?
If you genuinely have the best interest of the animal(s) at heart, you will not stop looking until you have found him another home. Please keep trying. Have you posted on Craigslist? Freecycle? The Pennysaver? Most veterinary offices have a bulletin board where people are able to post pets that they are trying to find homes for. . . you're sentencing this fish to death!
If your only option truly is returning him to WalMart, do it. At least then he has a CHANCE, however slim, of finding a proper home, and your hands are clean. At the very least, you're joining with others in refusing to buy, and returning, fish purchased from there, which will ultimately lead to the company losing money. When this becomes enough of a problem, they will shut down their fish department. My local Walmart no longer carries living animals. Return him.
Betta, shrimp, snails, a single African Dwarf frog - these are possible options that are better suited to a tank of this size - after you have found the Puffer a new home.