I'm pretty on the fence when it comes to the Cacodemon leak:
On the one hand, no one has really had major success in replicating the models in a working fashion, particularly the jiggle physics of Mallow.
On the other hand, things like Artsy Omni's Ray-Man leak and the Grinch leak show just how far some people are willing to go for their leaks.
On the one hand, fake leaks tend to go for the most fan requested options to get the most exposure, which certainly fits Geno and Doom Marine. Real leaks tend to be a little more grounded and have characters outside the Smash Bubble.
On the other hand, this is the game that gave us Ridley, K. Rool, and every character in the franchise. Fan service has been a major theme of Ultimate since the game's formal unveiling. Likewise, Geno and Doom Marine have plenty going for them, so seeing them join the battle isn't that weird.
On the one hand, there was that Greentext leak that predicted Goemon, Team Rocket, and Mallow as Mii costumes. The former two were far harder to predict than Mallow, so it's likely that was legitimate insider info.
On the other hand, even the most accurate insiders have had pieces of faulty information. We're still waiting on that Minecraft content, Vergeben.