Hi Dimitri,
My understanding for SRM and SUS integration, SAP provided out of box PI content, and you can easily configured and use, so it make sense to use PI.
while if SLC and MDGS is supposed to use P2P, again, if it was built that way, better you just follow it.
I used PI for SUS/SRM communciation and ALE/idoc for ECC to SRM connection in on one project. SAP interface can have multiple approach, really depend on which fits better.
Hope this help with your answer.
--Wenning
