Im playing around with ideas for a 10 gallon planted tank. This isnt so much a stock question, im just interested in looking at teeny fish lol. Idk if im even going to set the tank up. Depends on species availability.
Glass gobies would be the smallest I can think of, but I've seen other very small ones in my LFS nano rack. Scarlet badis or badis badis are also small. This isn't freshwater, but brackish(depending on where you get it you may have to start with freshwater and slowly transition to brackish), the freshwater flounder always caught my eye. It's also small and I believe you can have 1-2 in a 10.
Oh, and Pygmy sunfish are pretty awesome.
The smallest I can think of that are commonly available for aquariums (I know these aren't the smallest though) are ember tetras, dwarf puffers, sparkling gourami, maybe some pencil fish, and the various dwarf rasboras (galaxy, emerald, chili, etc.)