To be clear, PNSB do consume (assimilate) nitrite, but they do not perform nitrite oxidation. I don't see much of an issue with adding simultaneously, since PNSB love light and are primarily anaerobic, whereas nitrifiers tend to dislike light and are primarily aerobic; in other words, they occupy starkly different niches. That being said, there would be no issue adding the PNSB later, maybe after nitrate begins to appear (Rhodopseudomonas palustris, one of the two species in PNS Substrate Sauce, is a denitrifier). I wouldn't necessarily advise adding the whole bottle though, since each full dose of 1.25 ml/gal raises PO4 by ~0.05 ppm (just add the full dose, assuming very low PO4 level before dosing). Note that the nitrifiers require some PO4 for growth, of course. Also, at least some nitrite oxidizers (e.g., Nitrobacter) are cobalamin (vitamin B12) dependent and will exhibit very poor growth without some form of B12 supplementation.