In our lab, we routinely make pH 5.5 plates and our method is this:
1. Autoclave the agar, Y and P.
2. Make concentrated buffer and glucose, autoclave separately.
3. Add together after cooling, pour plates.
If your buffer is strong enough, it shouldn't be affected by the pH of the
YP mix. I don't know if this would work for pH 2.5. We found that the
agar liquefaction process only occurred when the agar was autoclaved at
the low pH, not when pH'd after sterilizing. Good luck.
UC Davis Microbiology