20l is clearly to small for any fish.
People here usually recommend 45l for a single betta but with a lot of maintanence, a 20l should be just fine for 1 betta.
Too small tank means the water get pollute much faster with fish waste.
For other fish.... Apart from the dwarf puffer, I don't think anything else would fit a tank smaller than 45l.
Even mosquito rasbora will need a 45l minimum as they live in school.
It's really great to see people actually pay attention to welfare of fish :) Many fish keepers completely ignore this