I did a 2.5 with a pair of Apalachicola pygmy sunfish (Elassoma sp. cf. okefenokee) and ghost shrimp. It lived on my bathroom counter and was quite nice.
The pygmy sunnies were ridiculously shy and the tank was creatively covered in algae so I didn't get to see them a whole lot but it made the few times I did get to see them that much more exciting and the ghost shrimp were actually quite fun.
Male pygmy sunny.

female

One of the pics I'm most proud of:

You could do the south-east Asian version with Dario dario and some Caridina or Neocaridina sp. too.
All I recommend is good filtration and 50% weekly WC's.