I didn't think a 2 gallon hex would work with the size of a Aqua Clear 20 HOB Filter. Look into dimensions. A 2 Gallon, you could keep around, 1 or 2 dwarf hermit, 1 or 2 snails, and the pom pom crab, or shrimp.
I think of it like this:
A tiny clean up crew
and a "bigger" animal, like a more expensive crab or shrimp compared to the clean up crew.
I would do a cool crab like the pom pom, OR sexy shrimp and the tiny clean up crew, with a few corals.
And as everyone says about pico tanks:
Watch parameters, make sure to top off.