There are only a couple pleco's small enough for a 20g and even that IMO
is pushing it. Bristlenose, Bulldog, and Rubberlip pleco's would possibly work. Oto cats and Nerite snails are a better choice with a much lower bio-load.
I feed mine a lot of blanched Zucchini. But they also like cooked, deshelled peas, cucumbers, and algae wafers.