i guess if i were you i would go with a school of tiger barbs and maybe 2 clown plecos. i would avoid the celebes rainbowfish since they do better in brackish water, and prefer to be in groups. a group of celebes would take up a lot of room in a 20H and you would not be able to do much else.
or, you could do the pleco with a group of tetras and a centerpiece fish like a dwarf gourami or an apisto (very cool looking fish). a lot of tetras actually stay closer to the bottom, so instead of a centerpiece you could also choose some livebearers like guppies or endler's (all males if you don't want babies) and they will occupy the upper portion of your tank.