I can't speak for the loaches, but a twenty is too small
For a red tail / rainbow shark or tigerbarbs. I recommend a 55 for a rainbow and a 75gal for a red tail. Either one becomes territorially aggressive as the mature and reach between 4 -6inches in length. A school of tigerbarbs are too active for a twenty... A 29gal or up is better.
75g Planted Barbs Denison, BlackRuby, Tiger, Albino Tiger, Green Tiger, Cherry, Gold, Checkered, Odessa.
NonBarbs Cardinal, Neon, RummyNose, Bloodfin Tetras, Z. Danios. 1RTS. Amano, Bamboo, Ghost & RCS. 20g Long Shrimp Tank RCS, Tiger, Amano.