Is it a 20g long or tall tank? With the long size you can stock slightly more.
Regardless, you could do something like this:
6-8 cardinal tetra
6-8 rummy nose tetras
honey gourami
german blue ram/bolivian ram/apistogramma (1-2 of one type under the right circumstances)
That stock is very, very similar to what I have in my planted 20g tall. Please note that this is also under the assumption that you have great filtration and will do very regular PWCs. Also note that baby RCS will be fair game for consumption, but providing you have a decent amount of coverage (driftwood, java moss, etc) your colony should be fine. I've never had micro crabs so I cannot really comment about those. Other fish you might want to look into are platies, pygmy cories, any other peaceful tetra or barb (i.e. cherry barbs, 5-banded barbs, etc), kuhli loaches, and so forth. A lot will depend also on what your LFSs stock. Good luck!
