Other tiny fish include Heterandria formosa, golden teddies, black chins, Characodon lateralis, endlers, guppies, and several others. I have only listed live bearers but plenty of egg layers also stay quite small.
Celestial Pearl Danio are my favorite tiny fish they are lovely looking and great little characters!
I think you need six at least otherwise I read they can be very shy. Not sure any more than that in a 27L.
I have a shoal of 9 and they swim happily out front with all the bigger fish.
I used to have a 28litre tank with a siamese fighter, 2x albino cory, a couple of algea eating shrimp, and about 5 neons and they were all good together