I dont claim to be any expert, but I have been keeping DPs for over a year fairly successfully. Just an idea, keep about 1 dwarf puffer for every 3 gallons. This is the planted tank forum, so I assume you will be planting the nano fairly well. As sad as it is to have such a small fish in a relatively (to their size) large tank, that is the best for them.
They are fin nippers, so its reccomended to keep them in a species tank, and also, if you want to have shrimp, dont spend too much money/become too attached; they will pick on/eat it. I have been lucky, after finding a mysterious ghost shrimp in my tank over 2 months ago, its still there, chewing away at the algae.
My 10gal tank has 3 DPs, 3 Ottos, and the ghost shrimp. There are tons of plants but its not over crowded.
Dwarf puffers also have a very unique personality; but they can be a pain to other inhabbitants. Ottos are pretty good tank mates, so in the 6 gal you could probably have 1 or 2.
Good luck in your puffer adventures, they are such a cool fish to keep in a planted tank. Infact they are what drew me to the planted tank scene.