I know ottos aren't puffers, they are however, potential puffer tank mates for only certain kinds of puffers.
Just because I can possibly have more than 1" of fish per gallon, I prefer not to test that very far, ever. which is probably good. When I get into marine systems, the inches per gallon becomes even less. Plus, if I stick w/ 1" per gal, I don't have to go through the trouble of measuring how many gallons the ornaments displace. But that's just me. I know people stock more inches in tanks. Of course, w/ fat, wide, full-bodied fish, the inches per gallon decreases, then as well.