Regarding that Mii Costume "Premium Costume leak", while I Mostly don't believe it, If for some reason it turned out to be true, I wouldn't be completely surprised and would understand the decisions
-All those amount of SNK Mii Costumes doesn't seem out of the ordinary. My only gripe is no Iori
-Sora, Crash, and Doomguy being Mii Costumes seems stupid, but i can understand why they would be. I think with the whole, bring game worlds together and what Ultimate represents about putting all these worlds together, Perhaps there was things that held these characters back from being Playable, or that other characters were more of a Priority over these characters, but Sakurai still wanted them to have a role in the game, hence the Mii Costumes
I Can especially believe Sora, Now Sakurai doesn't have to go through of potential hurdle of having to put in Disney Characters like Donald, Goofy, etc in order to best represent Sora, if Sora is just a Mii Costume. i mean yeah they could put in Sora without the Disney References as playable, but i don't know what Sakurai is thinking and perhaps he feels Sora as playable isn't right without the Disney references.
-Extra Costumes for the Smash cast has been a Popular idea i've seen, so it would be great to capitalize on that idea
-Waiting this long to Reveal Geno, while not the best idea, wouldn't be the first time they pulled something like this off (Isaac and Shadow can attest to that), and perhaps Geno got his mii costume upgraded in model