I assume that lava rock would be fine. I would have gone with base rock if I wanted to go that route. What I did was buy a little LR at a time until I had the aqua scape I wanted and more importantly the water to rock ratio I was shooting for. Whenever you add a piece of new LR there will probly be a mini cycle, but nothing you can't control with a water change. The biggest problem with adding base rock instead of LR (again, I assume this applies to something like Lavarock too) is the time that it takes to seed the rock. Lets say you have 80% base rock and 20% Live Rock, you're probly looking at several months before it is all active live rock and doing it's job. If it's all LR, as soon as your done with your initial cycle, your good to go. I would be cautious of any rock that isn't explicitly for the hobby, who knows what metals and chemicals are hiding away in there? One thing is for sure... if you don't put any Live Rock in the tank to seed the base rock... you won't get much out of it.
Don't worry so much about the skimmer while you are cycling, unless you just want to mess with it and learn how to make it work properly. Until you start putting livestock in, it's not going to benefit your tank.
Everything I've said is my opinion, and not absolute fact. I would research everything you ever read before you implement it.
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