it will not mix into the gravel, like different pebbles can be mixed together.
its like dust underwater and the slightest of movement can stir it up - so you have to place a layer of it at the base, and then cover it with gravel or sand.
also beware of algae problems, though I have not had that problem - many others have.
the lighting you need depends on what you want to grow.
most will recommend 6500K-10000K.
I like 18000K as all my plants have done brilliantly under that light - mind you, you pay a lot for those tubes.
if you have a LOT of plants, you don't need to siphon the gravel. just your normal water changes will suffice.
HTH