I can't speak from experience, but when I frequented Nano-Reef.com in the not-too-distant past, the general consensus that skimmers on tanks that small are more trouble than their worth, and instead you should focus on religious weekly water changes. The corals you listed don't seem to indicate a need for zero nutrient water, either. That being said, I don't see a problem using one unless it's grossly oversized.
The AquaC remora is always a popular HOB option, and they make an optional surface skimmer box. However, IMO it's expensive for what it is, and for every 5 people that love it, 1 or 2 can't get it to perform.
Another option, I have a used CPR Bakpak 2R skimmer w/MJ1200, including wood airstone and air pump (the 'airstone' mod), that performed admirably on my 100g volume system. Obviously it was undersized for that application so I just upgraded to an Octopus NW150, but on a 20g should be more than enough. The air pump is a Hagen Optima, and runs very quiet. If you're interested, let me know. (Looks like they make a
surface skimmer box for the CPR too, although I have no experience w/it).