I'd put as much macro algae in there as i could possible get, not Live Rock. That would be just like leaving the bio balls in there, it would collect crap and in time cause nitrate issues. Whereas Cheato would take down phosphates, ammonia, nitrites and nitrates. Ideal all around. No rock, no sand. Just let the cheato or Caulerpa grow out, this would be miuch better for your system.
Recommneded depth would be after you fill it half way, turn off the return pump and you get the back flow to fill the sump, this will give yoiu an idea how high to actually fill the sump, yoiu want it as high as you can, so that its still able to hold the back flow in case of power outtage. The skimmer you can always put on a plastic block to get it higher in there if need be.