Like Byron said, Discus prefer low lighting. They come from a habitat where they get little sunlight, and prefer this type of setup. When you keep Discus in a high light set up, they will be stressed out. They will still survive in a high light setup, but it will be harder to get them to breed and their lifespan may be shortened. The same thing goes for neon tetra's who come from a similar habitat.
On the other hand, if your going to go with CO2 then your going to need high lighting to balance the CO2 out. You can always go with a lo-tech set up without CO2, which would be ideal for the Discus. You can still get heavily planted tanks with a low-tech set-up, if you need an example just look at Byron's tanks.
In my opinion it would be best to either go with a low-tech setup or a different fish that is more tolerable to high light, but that is your decision to make.
I recommend you read this article that Byron wrote, it goes in-depth about how lighting affects fish. http://www.tropicalfishkeeping.com/f...er-fish-81982/
Good luck with your project