Sure. It's OK to mix substrates. In fact, you can also mix in a little gravel, because you don't need 100 percent clay based substrate. If the Eco Complete is still wet, it probably still has some of the beneficial bacteria that help start the nitrogen cycling process. As an alternative, you can use a cup of gravel from an established aquarium.
This website has quite a bit more about the nitrogen cycle, so you may want to look in the blog section. Proper cycling will help get a new aquarium off to a good start.
I forgot to say welcome to our forum, so WELCOME!