Anyone tried or has experience using Food Grade Phosphoric Acid as a PH Down?

there are plenty of options that don't add overlapping plant needs. white distilled vinegar is usualyl found in any household.. that's cheap and easy to use. ph-drift is not caused by the acid you choose. the acid you choose is not more stable than other acids. ph-drift occurs when a solution is not properly buffered. it is a sign of a shitty manufactuer of soilles/hydro nutes (not soil nutes, those are rarely ph-buffered)

yes, as long as it doesn't create an imbalance related to adding more P ...

It should work fine. I use a phosphorus ph down. I generally add it with a eye dropper.