Your question seems a little confusing. What size is your fan, and how many CFM's?
Personally, I would never combine my carbon filter with my inline fan for my lights. They are seperated, due to the reduction of CFM's. If you push or pull, you could be comprimising the air flow you need. If your fan starts with a fan, you may not be able to move enough air, and have heat trouble with your lights.
Just personally, I have a seperate fan for my lights, that push air. I then have a bigger fan, connected to my carbon filter that has its own ductwork. Much better for the system, but $ talks.
Well, now that I'm taking the time to look at your picture, it looks like the whole thing will be fighting eachother. It seems that your 8" will be pulling whatever your 6" pushes, and leave your lights out of the loop. It seems to me they need to be seperated.
Maybe someone else will have a different opinion, this is just my humble opinion.