you could put pretty much any fish that is big enough to not be considered food with a puffer, they are pretty mellow fellows.
But in a tank your size which is further hindered because of it's shape, you should stick with fish that max out at about 4"-5" if you want more variety.
Having said that, the only things that I feel would be decent tank mates in that size, but with the caveat that you will eventually need to rehouse them, is a lionfish and an eel.
They are all able to eat the same diet and on the same schedule and all are rather docile and not active swimmers so the limited area of the tank isn't as big of a consideration.
All this talk of what messy eaters they are is irrelevant, all fish are messy eaters, it's just a matter of how big the chunks they spit out are.
The way to manage water quality with big carnivores such as those mentioned is simple, only feed them once a week, problem solved.
Actually if you do it all in one day even better.
Feed them a big meal, wait about 2-4 hours and then do your water change.
Those guys do just fine with the occasional big meal as that is often how they feed in the wild.