Lucky day I think?? help with new Sump.

[mvallee]

Hoping I can get some suggestions on what I can do to maximize the effectiveness of my new sump. I am not new to fish keeping, I have been keeping cichlids for 15+ years and and just now making the leap to Salt Water, more specifically a FOWLR tank. I have done so much reading my eyes and brain hurt and so much conflicting info I am surprised anyone ever tries, I am starting with the basics filtration for my 180 with dual overflows I have decided on a center return with refugium on one end and Skimmer on the other end being fed independently so I can control flow to the refugium and both feeding into the center for return to the tank.
Today was my lucky day I think as I was in Petco buying a 55 to build my own sump a friend stumbled upon an acrylic Sump already setup and seemingly nice but a little different than what I would have built. basically the fuge is smaller than what I would think acceptable for a180 but the Skimmer inlet is larger than I would ever think necessary and must serve multiple functions, it was actually filled with what looked like dried short spaghetti so I am looking for suggestions on how to best utilize this 55 gallon sump/refugium.
Here are two images I took after cleaning it up, input on right and refugium on left as it was full of sand and small pieces of live rock.

any suggestions questions or input greatly appreciated.

 

[mvallee]

My files were apparently too large so here they are resized

 

[mvallee]

my real question is what do I do with that large section on the input side of the Sump, it can't be just for water in there must be some other reason for such a large area, miracle mud, live rock, some kind of live plants why would it be so large? why is the fuge so small (on the left)? would like to switch them up. any thoughts??? plus this is plumbed for an external pump but everything I read was internal pumps, should I just block off the pump outlet and go internal or does external have some benefit