Code (such as ASME B31.1, B31.3 or any other) would decide how to calculate the thickness of the piping (hence schedule).
Design pressure – You shall use in the thickness formula that you would be using.
Design temperature (or maximum operating temp) – would be used to select the allowable stress for the thickness calculations.
Corrosion allowance (and other allowances such as mill tolerance) shall be added to the pressure design thickness that you would get from your calculations.
And brother, normally we use the material classes for any job pipe material, schedule (thickness), corrosion allowance, type of welds, applicable code, pressure and temperature limits, flange ratings are mentioned. Check from your group.