I'm guessing that you have the reactor somewhere in line with a canister? Hard to know where to give pointers with so little info. Unless you have severe problems with leaf litter or other floating debris I don't see the benefit of a surface skimmer, personally. I was looking to go that way when I began battling surface film, but I tried air stones on a timer for 30 minutes during lights-off period, and the scum went away. Stuck with that since, and never went to the trouble of building a skimmer (the ones being sold by the various companies are way to small for true 600 GPH canisters).
Yes, it is inline after the canisters output. I wonder if its a matter of adjusting the skimmer. I use the skimmer primarily to increase oxygenation.