Well, none of it REALLY happen- actually nevermind that.
The expanded media suggests it is indeed canonical, and part of it at least happened, according to "The Five Doctors". So probably yes.
The Fourth Doctor was taken out of his timestream in "The Five Doctors" during the events of "Shada". So it is canon. Furthermore, an audio version complicates it because it features the Eighth Doctor alongside Romana II and K9 mark II.