If your plaster is fully cured a metal brush will not hurt it. In fact, on fully cured plaster a combo brush of stainles steel and plastic bristles is what you should be using for normal brushing.
As far as adding acid. I would add it a pint at a time for each 10000 gallons, wait about 30 minutes with the pump running, test your pH, and repeat until your pH is where you want it.