The potential problem (if it exists at all) is that rapid fix is ammonium thiosulfate based and Kodak selenium toner incorporates sodium thiosulfate (hypo) in it's formulation. I am not sure the effect of mixing ammonium and sodium thiosulfate.

I don't use rapid fix other then in film processing. I use hypo as both my first and second paper fixers. I have found that to wash or rinse between the second fix bath and toner is begging for staining. So I go directly from my second fixing bath to the toner. From the toner I go to HCA and from there to the archival washer.