I agree with all of what Matt brought up.
I also feel the need to bring up a few other series where this stuff is important: Transformers, Super Sentai (And Power Rangers by proxy) and Kamen Rider.
Transformers' canon policy is... well, completely nonexistent, to put it bluntly. Literally anything in Transformers is canon, as long as it is an official work. This ranges from the various cartoons, the movies, comics and its various crossovers. They have direct, plot important crossovers with Star Wars, G.I. Joe and Marvel Comics, and have done these repeatedly. Now, these crossovers aren't mutually canon - these events didn't happen with the actual crossed over series, but they happened with Transformers nonetheless.