Re: are casein and gouache "interchangeable"?
After the Casein has fully cured, areas with Casein in them will no longer be able to be reactivated with water. In fact, water solubility goes away in about a week, and sometimes in less time with Casein depending on atmospheric conditions. This can be viewed as either a benefit or a penalty, depending on whether you go back and rework your Gouache paintings often.
I really like mixing Casein with regular watercolors, and since both Casein and Gouache are fairly opaque, I am sure they would work very well together too.
Whether it's considered mixed media or not is up to the people judging your work. I do not see anything wrong with mixed media anyhow and wouldn't count it against you personally.