I remember that rumor. I don't think it was Tower of Smash, though.
Okay, have you seen the Duck Hunt trailer? 8-Bit DK(A.K.A. Cranky) turned into modern DK. This means that the DK in 75m is the same one you play as for the purposes of Smash. Or at least that's a fair assumption.
Anyway, I think I understand how Sakuria might be doing it. He doesn't want models of the same character appearing in a match as an NPC while the playable PC exists. This is why Toon Link is replaced with Alfonzo. Yet, DK and DH are not in those stages, because they're 8-Bit. So, this may mean that Ridley may still not be playable. I agree with you, keep in mind, it proves that a hazard and playable are clearly possible. However, I think the issue might be there's no one to replace Ridley as the hazard/assist. On the other hand, maybe a very different version of Ridley could come up. Possibly made a sprite like Mr. Game & Watch is.
Interestingly enough, Skull Kid has two very distinct models. MM has the mask on, with different colors and clown shoes. While TP version has different colors, no mask at all, a slightly different straw hat, and regular shoes. If he wanted to make Skull Kid playable, he has a workable model that is distinct enough. Never mind pulling a Alfonzo and disabling the AT while SK is being played as. So the real question is this; Is there an extremely different model for Ridley that is easy to tell apart from Hazard/Assist Ridley and the same for Meta Ridley? Since it's pretty clear they can't just replace Ridley with another character in the stage due to his exact design. If he just swept in and grabbed someone, you could replace him, with say, a skeleton doing it.(inb4SkeletonRidleyAsAHazard/Assist).