Mixing Aqua Soil and Eco Complete

IMO it should not harm anything. Depending of how much soil you have available, you could just put a layer of soil on the bottom and top it with Eco.
But maybe your LFS carries different soils? I've read somewhere that for example RedSea Flora Base is very similar to ADA..

I had around half a 9 liter bag of Amazonia left. It was barely enough to cover almost 1 inch of a 37g tall. My LFS has Eco Complete and I think they also might have Flora based stuff. I just wanted to make sure that it was ok before I mixed the stuff together. I didn't want to start off wrong right away.