Normally the returns are smaller than the drains. In my reef-ready 125 I have 2 corner overflow boxes with 3/4" returns and 1" drains.
The outlet on the Mag 9.5 is 3/4",why did you use 1.5" pvc
Usually set up that I have seen is that the fuge is fed from the drain line, though in your set up it shouldn't make much of a difference.
You need another ball valve on the return line going to the tank to cut the flow so you don't drain the sump dry.
You might want to look at Melev's Reef - Sumps & Fuge's
for some alternate designs for your plumbing.