I disagree. Stats wise, they're relatively balanced, so giving them a shared stats spread as a single fighter in Smash isn't too bad. Ninjara may lose some speed and Min Min may gain some length to her punches, but overall its not that far fetched.
This I agree with for he most part. Spring Man, Ribbon Girl, Min Min, and Ninjara do have very similar speeds. Not so much for the other character (they're often slower), but these four are probably the most similar in regards to speed. (I don't really know what ARM girth does so that wouldn't bother me either.)
Likewise, none of these characters' unique abilities would be strange to see the other characters perform. The ARMS character has to have a parry and multiple jumps because of Smash's mechanics, so that takes care of Spring Man and Ribbon Girl.
Nope. Spring Man's deflect barrier is not the same as Super Smash Bros.'s parry mechanic. It's something that happens after you stop charging, not shielding. In fact, ARMS also has a parry mechanic, it's just called something different, and is less noticeable (though more useful) when you pull it off.
Spring Man is an all-rounded character, so giving him Ribbon Girl's emphasis on aerial play would be weird.
EDIT: In fact, aside from Ribbon Girl, Dr. Coyle, and I think Mechanica, jumping is a bad idea in general, so Ribbon Girl's gimmicks wouldn't make sense on Ninjara or Min Min either.
Likewise, Min Min's kick can easily be worked in as a neutral or up air, and it wouldn't be strange to see Spring Man, Ribbon Girl, or Ninjara kick.
I hate the idea of the kick being turned into an offensive move. That's not it's purpose. And yes, it would be strange to see Spring Man kick; He's a boxer, not a mixed martial artist like Min Min. It also doesn't quite jive with the light and airy Ribbon Girl's personality.
The only slightly difficult one is Ninjara's teleport, seeing how that's pretty personal to his ninja identity, but it still wouldn't be that odd to see the other cartoony characters vanish into smoke.
Yes it would. There is no indication that they can do that. It'd be like if Donkey Kong, Kirby, Bowser, or the Ice Climbers started ninja vanishing for no reason.
In terms of having multiple ARMS, it's likely that a glove-type arm will be mostly used with maybe a curve-type arm or multi-shot arm as a special move. Based on what little we've seen, Nintendo seems to really be going all-in on the ARMS character's range as their primary gimmick, so it makes sense they'd want to use the most vanilla arm to show off this idea. This would basically apply to any character who got in, regardless of their personal abilities.
This I agree with. I doubt any character wouldn't have access to their default ARMS, but I don't see them allowing Min Min to use the Dragon ARM in her normals or anything like that.