You don't have to have a refugium. A sump is just another tank tied to your main. It can be under your tank or in another room for that matter
You tank will need to be drilled or use an overflow box. Water drains via the overflow to the sump and then a return pump sends it back to the main. Having a place to put equipment is good but more water volume is good as well.
I have LR
in my sump and I have a rubbermaid inside my sump that is my refugium. I have sand and chaeto in my refugium.
*180 gal Display, 100 gal basement sump, 33 gal refugium, 3x250 MH, 2x160 VHO actinics, zoos, some softies, LPS & lots of acros and other SPS.
*100 gal prop tank plumbed into main system w/ 2x96 PC lights and 1x150 MH,