i've got a 150 frag tank set up that has a large skimmer in it that skims next to nothing out of the tank whatsoever. the frag tank isn't plumbed into any other systems and is a stand alone. i probably have 350 frags in there right now, along with a lot of the parent colonies. again, i'm not getting any skimmate from my asm g4 (overkill i know) that's on it. the way the tank is setup, i can add another frag tank of equal size directly above it and have the above tank drain into the bottom tank, which this has a return pump to push water back up to the top tank...bottom tank will act as a "sump/refugium" if you will.
however, i do think that you should put fish in the frag tank. tangs will obviously graze the rocks/plugs/frags/etc for algae and will create lots of waste to feed your corals.