firstyou can pre-sex a plant almost anytime after the first set of true leaves come out, as soon as they show sex and you get rid of the males just revert back to your veg cycle.

now when i grow clones or seed i use the 18/6 but thats just to save a little on the power bill

any light can be used either hid or flouresents. im old shool so i prefere hid

as far as how close, as close as possibe without burning the plants. put your hand on top of the plant and if you get burned then your to close. you can put flourecents almost right on top of the plant but hid needs a little space.