Questions regarding the material selection for gas pipeline systems and fittings

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1. I am currently working on a project related to the renovation of long-distance pipeline stations. The existing station already has D219×5.6 L360M PSL2 straight-seam resistance-welded pipelines; for the renovation, I plan to use D219×5.6 PSL2 seamless steel pipes to connect with the existing pipelines. I would like to know what the differences are between L360M, L360N, and L360Q, and what factors determine the delivery condition of the pipes. 2. How should the material of the accompanying pipe fittings be specified? 16Mn? L 360? WPH52? Which one is correct? 3. 16Mn should be classified as a low-alloy high-strength structural steel; is it permissible to use GC1 or GB20801 in its place? It is strictly prohibited to do so, but in practice this is how it is done 4. Is it acceptable to use 20G material? Is it appropriate to create bevels at the weld areas and reduce the wall thickness? Is it necessary to use pipes with a yield strength of 360 as well?