Personally, I'd count on 1000gph +/- through the overflow. I'd run the water to about 1/2" below the top of the tank so just about to the top of the teeth.
Tank looks to be 1/2" material, maybe thicker if my eyes are tricking me.
T5 lighting would be fine, no problem.
Why would y need more room between the slots and top of the tank? (just curious)
James