For the 55gal sump, is it for the sump/return area and the fuge, or just solely for the fuge? You might want to see how much space you might have inside your stand, and if you could actually get the skimmer in and out, without the top of the sump being to close to the top of the inside of the stand.
Mag 18 might be a bit much. I have one on my 125, with a 1200gph hang-on overflow (flowing only about 900gph realisitically with the bends in the pvc), and the return pipe is over 4' long, with a spilt Loc-Line nozzle on the end, and it's still pretty restricted on the ball valve. If you have plently of PVC parts lying around, you could 'T' the return line off one or even two or three times. Nothing kills flow like doubling up the work the pump has to do.