I'd skip the tiger barbs, not because they aren't good and interesting fish but because they do get just about 3" long, and will nip the bejeesus out of everything else in a 20 gallon tank. With tiger barbs, I have been advised, they pretty much need their own swimming level (or to share it with fish even meaner than they are) and, in a 20 gallon tank, to an adult tiger barb, everything is their swimming level.
I have had good luck personally with cherry barbs, and I have heard good things about rosy barbs.
Also, as neat as Harlequins are, Brilliant rasboras are also very pretty. More of the classic minnow shape.
My personal preference is for black gravel. I just used eco-complete for my bottom feeder tank, and it's nice and dark, and has all the good nutrients for plants.