If your water is on the hard side, I would look at either Corydoras hastatus or C. panda; most other cories of a suitable size for your tank (C. pygmaeus and C. habrosus) prefer it softer. The hastatus would be the best choice; pandas get that bit bigger.
Then you could have; dwarf neon rainbows, Celebes rainbows, Celestial pearl danios, Endlers or guppies, glowlight danios, dwarf emerald rasboras; there'll be others.