My well water is 7.9 to 8.1, (seasonal) and I've been using General Hydroponics ph Down (food-grade phosphoric acid) for years. I bought a gallon two years back, I still have almost half, and I use it for every watering.

1 tsp per 5 gallon pail of water brings ph down to 6.8 and no, I've never had any problems. I do flush on occation, usually once during veg, once or twice during flower. (plus the final flush before harvest) I do this to prevent salt buildup from nutes, tho. Not the ph Down.

I don't use sulfur or CO2, so can't really comment on that.

Nutes with high salt content...?