The octo nw 150 is a good skimmer, but it will not handle a 180 gal tank. The nw 200 would barely handle a 180, and only if it was lightly stocked and if you did the mods to the skimmer pump.
I would recommend a MSX 200 for a 180 gallon light to medium stocklist.
Oh, and a coralife 220 has no shot at a 180 gallon, I had one before and they are not nearly the skimmers that the octo's are, and the MSX's put em to shame.
As far as over skimming, I dont think you can.....I have a MSX 250 on a 120 gallon, but it is very heavily stocked.