I have between 3" to 6" inches of laterite, capped by 2" to 4" of pool filter sand, in my 75G modified Iwagumi. It's made a perfect bed for a tank full of DHG, dwarf sags and lilaeopsis mauritana. Earlier on, I had some anacharis growing for combatting algae. When I removed it, each stem had roots over 5" long into the laterite. Great combo for root ferts, root roids, gelcaps, etc.
I have FSS in two tanks and it has grown everything I've stuck in it. I do use plant weights on stem plants, as they will float easily at first. Even after rooting, FSS is not the most secure substrate for large stem plants. It is great for DHG, anubias, mosses, etc, though. I've never tried to use root ferts of any kind with FSS; liquids only.