Adding something like Flourite will help feed the root systems of plants but should be added when first starting the tank (ie before water is added). It can be added afteer the fact but is often messy and the fish should be removed.
I personally have not added anything but I do not have highly demanding plants - some crypts, java fern, java moss and a amazon sword. Eventually the fish waste will form a soil in the gravel I use.
As long as the roots can root through the substrate, you will be fine (pun intended). I use flourite but I have alot of heavy root feeders (swords). Any gravel will work (for the most part).
Really what substrate you use depends on your plants. As TKOS states, he has mainly water feeding plants and they dont care about the substrate. The one sword is a root feeder, but I am sure it gets enough becuase it is the only one there.
I have 40oz of First Layer Pure Laterite underneath the normal gravel. The rooted plants seem to like it. I also have stuck with undemanding plants as they are less worry.