I also got PCC from Malaysia, and I didn't submit anything from CIC.My brother got CGC from Malaysian consulate general in Singapore. The whole process just took 2-3 weeks.. Malaysian Ministry of Foreign Affairs has become quite efficient, I have heard..I couldn't submit my application due to some other missing documents though again waiting for ITA.
By the way, I didn't send them any letter from CIC to show that I have the ITA and I need the CGC for immigration. I hope mine is still valid for Canada and IRCC accepts it when I submit it after the next ITA.
As for validity, as long as you don't currently live in there or haven't visited Malaysia after your PCC was issued, you don't have to worry about it and can still use your old PCC.