Personally, I think slopes in tanks are far, far overrated. The sole exception being an Iwagumi type with heavy rock work. Otherwise in a heavily planted tank I can never really see if there is a slope or not. Therefore, I say, heck with the slope! I'd shift some of the 3" in back, forward. Giving you a total of 2 1/2" all around, I think this would be plenty enough substrate. All Diana Walstad recommends to cap soil substrates is one inch anyway. I have quite a few smaller tanks w/ just 1 1/2 to 2" inches of substrate, and these do just fine.
The problem with adding another, either front or back, is they will eventually get shifted and mixed. I tend to think of CQ as being very solid black and a tad shiny. The black Flourite is more dark gray and very matte in coloration. I'm not sure if blending them will look nice or not.
Before ever dumping it together in a big tank, I'd make a sample. Mix them up and wet them, place in a small clear bowl*, then slowly place it into the aquarium submerged (very careful not to spill it). You can then see how it would look under your actual lighting. If it's not looking good, you've not spoiled your whole tank.
*The bottom cut off a 2 liter clear soda bottle will work great. Not filling your sample up to the top, will help keep it from sloshing out.