Yeah, maybe a taller piece of driftood that goes nearer to the top of the tank, they really do enjoy swimming around and through things, constantly exploring. Bumblebee Gobies are hit or miss with puffers, knight gobies should be fine as they are much bigger. Ive kept 3 fig 8's with both in the past. Mine are in a 45G too, though mines more cube shaped and a bowfront.
One imprtant thing is to feed them plenty of hard shell foods like pond and ramshorn snails. Its easy to setup a snail breeding tank. If they dont chomp on hard shells, thier teeth will grow to the point where they will bridge from the top to bottom jaw and not be able to eat. In that case there are some good articles on how to trim puffers teeth which I have had to do with one of mine. The only 'dry' food I have had success with is Tetra Sun Dried Baby Shrimp, and now that they are bigger, Tetra River shrimp. But the majority of thier diet should be live snails.