Everything released to the public counts. All of it. Every episode, every movie, every comic, every novel. If the owner of Star Trek sees fit to put their stamp of approval on it then it counts.
However, there are contradictions. How to handle those? Simple. Just watch Parallels. If an episode contradicts something that has been seen before then we're actually seeing Something from a very similar but different dimension.
Gorn look different in Arena, IAMD and STIX? Different dimensions.
Was Fermat's Last Theorem ever solved? The Royale says no yet Facets says yes. Different dimensions.
Kirk's middle initial R or T? You get the drill.
Don't let anyone tell you something doesn't fit or isn't canon or is wrong. It's all good.
It's all part of the canon. The real question should be, it is part of the continuity?
As for the "stamp of approval". That can be removed on a whim. What's okay one day can be trash canned the next.