For a good sand-sifter, you could get a conch... and some shrimp-gobies will sometimes sift sand. As a bonus, the shrimp-goby / pistol shrimp symbiotic relationship is fun to observe. I would avoid "true" sandsifting gobies in a 20g, just because they can have a very specialized diet and often perish in small or under-established tanks once the sandbed flora and fauna is depleted. There are snails that can help clean your sand as well, like the nassarius and cerith.
Stocking fish in saltwater is a bit different than fresh. You will be stocking much lighter than a 20g freshwater tank, depending on your plans for filtration. For example, for a 20L with a normal HOB filter (no sump) I wouldn't probably suggest more than 2-3 fish total. You also want to avoid stocking multiples of the same species in most cases. A percula or ocellaris clown would work, maybe a basslet or firefish (I love the yellow assessor, it's a bit crypic but swims upside down which is kind of cool). You could also get away with a pygmy wrasse (wetmorella sp). If you do choose a wrasse or dartfish, a tight fitting screen top is required to prevent carpet surfing.
There's other good ones as well, you could check out the "nano fish" section on Live Aquaria for some ideas, just make sure to research new additions thoroughly.
Welcome to the salty-side :)