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.

Most fixatives are resin-based, dispersed using alcohol. The latter will evaporate quickly, so you're basically looking at a resin/resin bond, but I don't know how different resins work at each other chemically. I can imagine the fixative resin won't play nice with the water-based polycrylic.

Apart from not using fixative and spraying the polycrylic so you won't smudge the chalk to begin with, I suggest making a fixative yourself using shellac (mix with isopropyl alcohol, 1:8 respectively), and applying it using a spray bottle.
Shellac bonds well with most coatings, so it should as well with polycrylic (see also here).