You may have to be careful about the Red Tail Black Shark, even in a 20 gallon. If there will be any real competition it will be from this fish because they're highly territorial. The Flash will be no real problem because, although it gets to a slightly larger size than the Zebra it doesn't have the volatile temper of the Shark. They seem to be happy with smaller territories, whereas the shark may take up a much larger one. Make sure the oxygen level is high for the plecs as well.
As to feeding, try 'target' feeding using defrosted bloodworm and a small syringe/pipette or baster. Load it up, get your hand in the tank and introduce the food to wherever the plec is residing. See how you get along.