400w should be enough light. I think that montis prefer to be mounted on a rock though. Thats only because I've never seen one in the sand before.
I agree with this, you need to get water flow AROUND the coral, not just over it.
Good luck with the system, I was skimmerless 2 plus years and now i run a small skimmer. In a mixed reef it would be a good idea to run carbon to clean up the toxin warfare going on.:happy: