+1 for the Puffer
As cute as they may appear, their appetite supersedes their desire to play nice. When I was a teenager, I worked at a lfs where we kept a tank of them in a brackish water. Against advice, customers wouldn't believe that they had a mean bone in their body and would insist on buying them to put in with mollies and platys. To convince them, and I'm not proud of this, we would put a feeder goldfish in the tank that was even larger than any puffer. They would attack it like a pack of wolves and devour it within moments. There are undesirable factors here such as too many puffers in one tank and most likely a bit underfed which is common place in a small mom and pop pet store. The point was clearly made to the customers which did save countless livebearers. In a saltwater tank, there are many practical applications where the puffer would be just fine. Your tank, in fact, may work out in the long run but keep the puffer in mind when fins look go missing after lights out.
Good Luck
Mike