The least aggressive puffers would be South American puffers. They are the only oddball puffers since they need to be in groups to live long healthy lives.
These pure freshwater puffers each require 15 gallons and only grow ~3inches. The only hard thing about these guys is that you will be required to manually cut their teeth at least 2 times per year. I'm told that it gets easier after a few tries.
You could put in some low profile fish in with these puffers. Maybe some a group of otocinclus or something that keeps to themselves (won't look at the SAPs). SAPs will nip fins of the fishes that get in their way or when they get really hungry.
I'm trying to set up a large tank for 6 SAPs.
Oops sorry I just realized that this is the SW