Good questions...this is what I did:
I have been using PI for many years...since version 2.0. I love it. I set up the catagories in Outlook, then I sync'd it up. Everything worked fine. I always work from Outlook because 1) I type faster and I write, and 2) I'm lazy.
I have never tried doing it from PI first. Changing the catagories in Outlook first, worked for me. Then whenever I assigned them to a task, it sync's up without issue. What version of PI are you using?