Minimum Gap Between Temperature and Pressure Tappings

The minimum distance in the pipeline would be where the two tappings are on the same circumference of the pipe, i.e. zero spacing along the length of the pipe. Without knowing some dimensions, this is best addressed at site.

You state pressure measurement, which implies a simple tapping into the line.

The thermowell may briefly change the flow characteristic in the pipe, depending on the ration between it's length and the pipe diameter, but the pressure will remain constant. Try and mount the thermowell downstream of the pressure tapping if possible.