I dont think that there would be any problem at all. Are you going to mix them together or use one on top of another? If you are mixing them together you might try mixing a small ammount in a container to see if you like the look of the mix.
The point, for me, is I like the darker coloration of the eco and it is less messy than the flourite. I am thinking of setting up a new tank and have a bunch of flourite which I don't want to waste, therefore the adding of more gravel in the form of eco.
Iris, don't think if you use eco, or any other kind of gravel short of a layer of soil, that the plants needs will be met. You have to add nutrients to the water coloumn with any commercial gravel I am aware of.
Are you going to layer them or straight-up mix them? It would look kinda funky if they were mixed, I think.
I've heard that Eco-Complete comes with a thin layer of organic material that settles around the bottom, though I've never tried the stuff myself. If you're adding the fluorite, you might possibly want to add a tiny bit of boiled peat moss, because in terms of volume the organic material from the Eco-Complete will be less. I don't think it will matter much, though
Keep in mind that it's impossible to separate the two after mixing Other than that, I think it's a great idea.