Flourite (I assume you mean Seachems substrate, not the mineral
) doesn't contain much in terms of nutrients. There are some micro nutrients and ferric iron bound up in it that plant roots can extract over time, however, for a normal medium light planted tank's purpose there is not really enough to say it will supply nutrients.
Clay substrates like Flourite have a high CEC (cation exchange capacity) which means they can bind
nutrients very well, which makes them valuable for our planted tanks. As detritus and fish excrements decay, released nutrients will be bound in plant available form. Same of course for root tabs/gel caps/fert stick additions.
So, in a way, with time Flourite gets better, assuming you do add nutrients to it via detritus, direct fertilizing, or nutrient additions to the water column. If you have heavy root feeders like Swords and Crypts for example, you want to supplement their growth with nutrient additions.
Covering it with something a little smaller isn't really necessary. If you think of sand or such, keep in mind that as you plant and un-plant over time the finer substrate will move down and the Flourite particles will end up on the surface.