A sump is a place to keep equipment you don't want to see in the display and it allows you to have more water volume which is always good. It can be as big as you want. Some make it as big that will fit under the tank in the stand. On my 75 gal
, I started with a 20 gal
, then went to a 30+ and now I have a 75 gal
A refugium is typically for plants but is basically a low flow area that has sand and/or rock, plants and is lighted. The plants, rock and sand add for filtration and in some instances, the 'fuge adds more water volume. Bigger the better. A lot of people incorparate the fuge as a small part of the sump.
I would put the skimmer in the sump. You don't have to separtate the skimmer, etc but you can. Depending on the size of the sump, separate sections might be needed to prevent micro-bubbles.
Filtatrion is the LR
and/or refugium. The sump just adds water volume. You can add rock in your sump for added filtration though.
There are a ton of designs out there so do a search on here or on google and you are likely to run into some pics. I used to browse through the photogallery when I was looking for something too