eco complete IS "fertilized" substrate...
no need to mix it with anything else
Sorry, but no. Eco complete has a very small amount of trace elements, but no macro ferts. very, very little potassium, no phosphate, no nitrogen. I could grow most plants in inert sand in a well-stocked aquarium, being able to grow plants in a substrate does mean it necessarily has fertilizer.
Don't believe me? check the back of the bag.
Eco-complete also won't buffer like Amazonia will. I think after reading boringname's link (very cool btw) and my own ancedotal experience with big eco complete pieces moving up that the Amazonia should
stay on the bottom. I would imagine that the Amazonia is of a lower density. You could also dust the bottom with peat, to get a similar result. (without the added fertilization though)
Also, Eco complete will work great for foreground plants, just let the more delicate plants (HC in particular) get well rooted before you add lots of shrimp or bottom feeders. The picture in my avatar is HC and downoi grown in Eco-complete.