As has been pointed out, that OF is not very well suited to seedlings. they won't need any nutrients for a couple of weeks as they "feed" off the cotyledons. (The first "rounded" things under the normal looking leaves).
Are you gonna go with chemical or organic nutes? Since organics takes a bit to get your mind around, I'd suggest chemical at first. Just be careful of the concentration. Chemical nutrients can burn your plants. In this case, less is more. Start out at no more than half strength. Eventually, you'll want to take a look at organics. It's a whole lot more forgiving, and your weed will taste better.
The floros are fine for now, but after they get a couple more sets of leaves, start them under the 400. I'd wait until they have at least three sets of leaves before I transplanted. Cannabis tolerates crowding quite well.
As for how far from the plant, the closer the light, the tighter the internodal spacing. Put your hand just at the top of the plant. If it doesn't burn your hand, you're good. I had one Sat grow up into my reflector 12" from the 400w bulb with no ill effects. KEEP an oscillating fan on them to build stem strength, and circulate fresh air.
Thats about it. Remember, there is nothing mysterious or arcane about MJ. It's a plant, nothing more. You are more likely to kill them with kindness by over doing things.