I don’t use mediums so I am unsure if blending the gloss medium with a heavy body will create a soft body consistency. My educated guess is that it will not. Do you absolutely need a soft body to complete this portrait and this course? No, but I invite you to try it if you can.
I started blending with soft body paints (whites and blacks) because I wanted to use less water to extend the life of the painting. I ENJOY painting with soft body paints and using it as a sort more fluid consistency.
Removing the the aspect of paint consistency from the question, I prefer a matte look to my portraits and I don’t use any mediums. That doesn’t mean you can’t 🙂