Mixing the two will work, as long as you cap it with something. But I don't like MGOPS. It's messy, and every time you pull up a rooty stem, you'll get a bunch of floaters. I have it in two tanks topped with Flourite Black Sand, and wish I didn't. You know how, in making MTS, you soaked it and scooped out floaters? If you did that with MGOPS, you'd end up removing 75% of the mass of the bag. No joke.
Still, if you want something under an inert substrate, it will work. You could just go with a really thin layer of MTS, but I don't know how effective that would be over time.
My suggestion would be to mix it with regular soil, the same stuff you used to make the MTS in the first place.