Anytime there are plants in the tank you should be dosing ferts... As for the lights I've seen people set them onto plastic egg crate directly on top of the tank, and I've seen them raised from 2" - 12" above the top of the tank.
I would suggest suspending the lights 4" above the top of the tank to begin with, and then you can see if you notice any changes to the plants... If they looks like the light is sort of burning the tips of the plants, you can raise the lights higher... If it looks like the plants are starting to take a dive you can always lower the lights...