you don't need to worry about high pressure- your pressure shouldn't be over 20-30 psi, and those gauges are designed for pressures up to 125 or better...
Old steam pipes were put together with threads, and had 50 psi at 250-300 degrees!
if the previous owner glued the pvc elbow over the existing port, I would make sure to leave enough when you cut it to attach another piece- but cutting and fitting in the adapter is a good plan