This! This right here, is a question that we need an actual, official, conclusive answer to. The policies no longer state any specific rules about the content of article pages. (We used to have standards for multiple types of pages: Characters, locations, objects, etc.)
As far as I know and everyone else is concerned, "Mobianization" (as I'm going to call it for now,) is technically allowed, because it has a significant link to the Sonic fanon. If you make like a Cirno the Chao, you get a pass. But If you were to just straight up bring Cirno in her entirety, then we'd have a problem. Even if you described her being on Mobius/Earth/Sonic's World, that probably would not be enough to justify a character page for her.
And yes, I'm going to use this chance to take potshots at this category that has grated on my nerves so badly for the past however many years it's been: Category:Mobianised Pokemon. Take a quick look through there and you'll see what I mean. All but one of those pictures at the top just straight up show different pokemon--at least one of which was literally ripped from the pokemon wiki, and we let people get away with this! Then when someone decides they wanna do the same thing with Ben10 or Jimmy Neutron or whatever everyone flips out. I never got this, and I can't help but express my annoyance now that this question has been brought up.
Adding up to Smash's point, I feel the need to bring up some of the articles that this guy made. As far as I can tell, they are literally just characters from an entirely different series that I assume were brought to the Sonic universe for whatever reason, assuming there even is one.