Too Many Layers?

I'm narrowing my substrate decision for a 21 liter freshwater planted tank. Here's the latest:

1. Base of PowerSand Special (Small or Medium)
2. Seachem Flourite Black Gravel
3. Cap of Seachem Flourite Black Sand

I'll probably sell what I don't need. The Seachem products come in 7 kg bags, I would keep probably 2 kg, sell the rest.

If I added some dry ferts + bacteria to the gravel layer, could I eliminate the Power Sand?

This link summarizes the many substrate options and was a great help: http://www.ukaps.org/index.php?page=guide-to-substrates

 

I really want to try the worm casting method plus some additives. Do they smell? They're definitely cheap and available. I'd mix with the black flourite sand (bfs), some Osmocote Plus, and something like leonardite then cap with the bfs.

No Power Sand, no regular black flourite. Just two layers, really - the nutrient layer (worm castings + additives + bfs) and the bfs cap.