PEX Pressure Rating

PEX is generally rated at 80 PSI for 200F:

http://www.nibco.com/PEX/PEX-Tubing/PEX-Potable-Water/

Hydronic Heating Applications

• This appears to say that if pressure is less than 80 PSI, then a pressure regulator is not necessary- https://www.nachi.org/forum/f22/max-pressure-cross-linked-polyethylene-73346/.
• PEX appears to be compatible with standard P&T relief valves - [1]. But this does not say whether the P&T valve can be connected directly to the PEX, or only upstream of it, prior to the PEX transition.