To midnight: It was about half, two week old rock and the other half, new rock never cycled (this is the 40G breeder), just fresh into the local store. The scrubber will take care of the nutrients so you can be at ease about that.
My 10G was filled with rock from the local that was two days old and the scrubber sucked it all up as well. As far as I can tell the amount of new rock does not matter as the nutrients will be eaten by the scrubber. So as long as the screen can handle the load, you should be OK.