Though it's also the simplest one, this can be the hardest part of our canon policy for new editors to grasp, but… yes.
Everything is canon.
Okay, not quite everything, but a vast majority of media
related (even tangentially) to Mickey Mouse, Scrooge McDuck & company are considered canonical on this Wiki. Is it kinda ridiculous that we're seriously proposing Who Framed Roger Rabbit, The Life and Times of Scrooge McDuck and Treasure Island all happened in a single narrative universe? …Of course it is, but that doesn't mean it can't be true. After all, you know what else is ridiculous? That we're having this conversation about a cartoon duck who wears a top hat and swims in money.
There's a practical reason for this: for us to function as a unified Wiki documenting all the adventures of Scrooge & Company, we have to give them all equal weight, and if possible try to fit them all together.
Do as you please in the privacy of your own headcanons, but that's the way we do things around here. Keep it in mind. See below for a few special cases and exceptions, and see over here for how to handle conflicting canon info! Happy editing!
WHAT ABOUT DUCKTALES 2017 AND SUCH?
Ah, good question. I see you're paying attention. It would, indeed, be ridiculous to try and fit DuckTales 2017, and other "reboots" like Kingdom Hearts or the Disney live-action remakes such as Dumbo, into the main continuity from which they are intentionally distinct. And it would be equally ridiculous to declare such huge swathes of the Disney Comics mythos "non-canonical".
Therefore, even when the official media don't explicitly tell us that, we consider that these media take place in parallel universes, and treat their events as valid in this parallel universe. Thus, we create separate pages for the versions of the characters seen there; they're, as far as we're concerned, different people. Della Duck never adventured with Scrooge, nor get stuck on the Moon; that was Della Duck (2017 Continuum), the alternate Della from the DuckTales 2017 universe. Got it?
SO WHAT ISN'T CANONICAL?
That, also, is a good question. Well, anything intentionally non-canonical, for a start. We don't necessarily need a statement in that direction to establish such intent; it's pretty clear to anyone with a brain that Don Rosa didn't mean for Return to Duckburg Place to be taken seriously as part of the Duck universe's history. It's also self-evident Life is Full Of Adventures With Aflac wasn't, as such, meant to take place in the world of DuckTales 2017 on any tangible level.
If you visit the Scrolls on how to handle conflicting info, you'll find that we usually wriggle out of contradictions, more or less elegantly. But there are also some things that just don't fit, whatever their creators intended, though they're very few and must be judged on a case-by-case basis. Rest In 'Pieces', for example, ends with Horace Horsecollar dying and everyone celebrating, so… yeah.
WHAT MAKES A NON-CLASSIC-CHARACTERS-FEATURING STORY CANONICAL?
It's true that a pretty wide variety of… stuff… is considered canonical on the Scrooge McDuck Wiki. Say, Aladdin. Or Gold of the Champignacs. Stuff that doesn't feature Mickey Mouse or Donald Duck or anyone at all from Calisota. Why? Simple: crossovers.
We consider that when a crossover story established another narrative series as happening in the same universe as Donald & Co., we therefore cover that series. (This is one-step only, mind you — if that series crosses over with a third one, the third one's not necessarily canon to us. Unless all three series involved are owned by Disney, in which case, go wild.)
On the other hand, if the crossover reimagines the characters to fit them into the Disney universe, or is done through dimensional traveling, these are not grounds to include the other property as canon. (In the latter case, simply cover things directly relevant to the crossover story, and link to the other franchise's own Wiki for a wider picture.)