Canon is all about trying to avoid obvious fatal contradictions in the ongoing plot and back-story.
It is a noble intention, but ultimately futile when you have such a long-running series that has been kept alive in various forms of media and by many different people.
I admit it, I do tend to think of the original sets of books as sub-canon, especially now they've been plundered for plots. But Russell T plays fast and loose with his own storylines, and likes winding up the more, erm, uptight fans, so I'm guessing anything goes now.