I too have a 20g tank as a sump/fuge. I don't have a skimmer ( my tank is 30g), but I think it'd be worthwhile to have one now that I'm upgrading. If I keep the 30 and set it up instead of going with a 55 like I'm planning as a second tank, I'd still get a skimmer.
Mine goes something like this: drain (from display)->bioballs->baffle->baffle->refugium (aragonite....should've gone with a live sand DSB ..... so do a live sand DSB!....some live rock, cheato, and mangroves)-> baffle-> sump.
I'm looking for a 30 or 40 long, about 12" wide that I can use as a sump for my 60g setup, and I'll have the drain from display into a baffle, skimmer, baffle to live rock rubble, then fuge, maybe another baffle with LR rubble for some more copepod growth, and sump.