If you go with puffers you'll be limited with tankmates, though you could probably easily put 3-5 dwarf puffers in a 20L.Puffers, Neons, Gourami's, Harlequins, Barbs, tetra's.....please give me some advice.
Neons or Harlequins (or any other small schooling fish) with some Cories would be a good choice.
If Barbs, you'll need to stick with pretty small ones, like Cherry barbs. I wouldn't put Tigers, Odessas, etc in a 20gal because they'll get too big IMO.
A single or a M/F pair of dwarf gourami would work. I wouldn't try 2 males, though.
Personally, if I had a 20L to stock I'd get another colony of wild-type bettas, probably Betta macrostoma.