I guess you don't need, NEED a skimmer if you do strong weekly WC's. I started with a 20g tank, but immediately got a rated 75g skimmer, and a 10g sump to put it in. If you only have a display 20g, now there are some smaller surface skimmers, and HOB refugiums, and skimmers, and combinations of the two which you might like. CPR makes a few models.
Honestly what normally happens is that with a smaller tank you want to keep from having alot of equipment hanging on the back, sides, or cluttering up your display. Just do a little research, and then you can make a stronger choice on which direction you'd like to take with your piece of the ocean.
