Any reason in particular you are getting the 400w fixtures? 175w or 250w will work fine and cost much less to run. Also, typically 1 MH
bulb will cover a 2' sq. area. A 6' tank should use 3 bulbs.
1- bulbs should be at least 12" above the water.
2- sure, I use basically the same kind of fans in my canopy, but I have both pulling the hot air out tho....and I only use 175w MH
. Those 400's will produce a ton of heat.
You can check out my build thread (link is below). I have pics of the canopy I built in there.