I used the clear poly acrylic, not the fixative. I had it dry for 24 hours and then added the thin film of walnut oil. From there I just applied the WSO in the same layer fashion that I do when working on any canvas. The painting is now 3 years old and has no problem that I can see.
I have used fixative on pastel paintings, but never used the clear acrylic over that because I always framed my pastels after matting them.
The best I can say is to experiment yourself and perhaps be pleasantly surprised.