Ok... if you're planning SPS, scratch the fuge entirely. A small part of a 20g won't do a thing for you.
I have a 20g sump on my 75g. I run an ASM G3 skimmer, with a filter sock on the return and one on the skimmer output. I have a reactor for phosphate remover, and a kalk reactor through auto top off. Toss my return pump and a few probes in there (my heater doesn't fit), and you won't be able to squeeze a paperclip in the leftover space.
Fuges don't belong on reef tanks unless they're set up properly. You can't set up a proper fuge in your sump. If you really want a fuge, use a separate tank, with a very slow feed pump and a gravity drain into your sump.