Actually, I was going to post that info a couple of days ago but I was unsure on how smoothly you wanted the rack/gear to run.
A metal rack is usually pretty smooth because it’s completely machined. Whereas, the injection molded rack, while having a nice smooth surface finish because of the molding process, the tooth geometries may be off a little bit not giving you the silky feel of a machined rack.
I’d suggest as a first round, using a plastic pinion with the plastic rack, but if that doesn’t have the feel you’re looking for, then I’d try swapping out the plastic pinion with a metal one.