Selenium toning like other chemical processes leaves a stain on plastic trays. But I use the same trays for various purposes. Ive never experienced any ill effects due to the residue. I believe that these chemical residues are essentially the byproducts of chemical reactions and for the most part not the active chemical. Therefore, its very unlikely that the residuestains reallywill have any effect on subsequent processes.
Whether these stains are on plastic trays or the interior of plastic tanks shouldnt matter. Ive used my Jobo 8 x 10, 11 x 14, 16 x 20, and 20 x 24 print drums to develop print RA4 and I used to do Fujichrome Type 34 (same chemistry as Ektachrome 22 and 23 papers), B&W print developer, fixer, Heico Permawash, wash and Kodak bleach-redevelopment sepia toning all in the same drums.
I also develop, fix, hypo clear, wash and occasionally selenium tone negatives in the same Jobo film developing tank. Ive never given any of these anything more than a water rinse afterwards and Ive never detected any problems. The development and other steps all seem to proceed normally without any unwanted side effects.
If there are any residual chemicals remaining in the stains I suppose that there is simply too little to cause any meaningful effects. You claim that wash aid is not recommended for Jobo tanks? I read all the accompanying information supplied with my tanks and drums and dont recall such a warning.
I've used Jobo tanks, drums, and reels fairly regularly as described since 1988. I haven't seen a problem yet.