I got a response from from both CIC and the Canadian embassy in Mexico to send them the documents for my Mexican PCC. It seems we can send them the documents right after we submit the application and it will speed up the process. The 'Request Letter' from CIC is just a procedure for any missing or unclear document, so there is no need to wait until the request letter to get generated.
Adding this info to help the folks in future, without this forum I don't think I could have made it through. Thanks to all.