dockmas...not all lava rock is reef safe and unless it was obtained in a good, reputable marine fish store you may want to look into that further before you go ahead and use it. Limestone is frequently found in reefs but, again, unless it was obtained specifically for a reef I'd hesitate to use it.
But, if these rocks were obtained from a reliable source for your marine tank, you can put it into the tank now and, with it, the live rock. By putting the live rock in with the inactive rock, you'd be accomplishing a few things....First you'd initiate a cycling of the tank which is precisely what you want to do. Also, you'd be "seeding" the other rock since living organisms will spread into the "non-live" rock that's in there with it (making it "live rock") and coraline algae will spead onto it also. That will take a while though, and things will generally develop faster with the more live rock that you put in there. I'd recommend that you have a total of from about 50 to 70 pounds of rock total in the tank.
I ran my skimmer from the very beginning and I'd suggest that you do too. You should begin testing so you know how the cycling is progressing as soon as the tank is full of water and has the live rock in it. It's important to test for ammonia and nitrates along with the other things you are testing for, with an emphasis on the ammonia and nitrates for the cycle. You didn't mention testing for alkalinity and that's something you should watch consistenly when you run a marine tank and keep it in proper measurements. You don't mention coral, so you needn't place too much emphasis on testing for calcium and magnesium but if you do intend to have a reef in that tank, you will have to test for those, as well. You might find the info about water qualities here useful:
http://reefkeeping.com/issues/2004-05/rhf/index.htm
Also, if you intend to have a reef, in fact, even if you don't and will keep fish only in your tank, I'd increase the flow in the tank with at very least, another Seio 620. That will keep the rock cleaner and healthier so it will be more in doing what it's intended to do in your tank. Hey, good luck with this.
