Can I use polycrylic over a fixative?
Yes, it should work, though I recommend testing the combination on another piece to be certain.

One of my hobbies involves "painting" resin pieces with chalk pastel; to both protect the surface and provide tooth, and to fix the pieces in place, spray finishes are typically used on the piece. These aren't necessarily sold as "fixative," but they behave in a very similar manner. Finishing touches are then added using a high-gloss varnish over the final matte spray layer, and there are no issues with doing this.

Since the materials you're using (ground, spray, and varnish) are slightly different from the ones I use, however, I would recommend a test run before committing to the final piece, but based on my own experiences, I don't see any reason why it shouldn't work.