Keep the footprint small, as it's a small tank.
And remember that just because you can
keep it in a small tank? Doesn't mean you should.
Puffers typically need 5-10gal at a minimum to thrive.
Even Boraras brigittae need more than 2.5gal, though they can certainly live and be okay in the tank. While a dozen of them would still look small in the tank, they do best with more swimming room.
Your best bet is shrimp.