As Sakurai said himself, he and his team could do it if they really put a big effort into it. There's no denying Ridley is one of the harder characters to pull off, his structure is really what could make it hard, and it's really the one and only reason he wouldn't be included since he has everything else going for him. However, we're talking about the man who made Pokémon Trainer happen, and this time with a bigger and incredibly experienced team.
And I'm well aware that Ridley's size is part of his character, but he doesn't need to be several feet bigger than his opponents. To seem imposing he simply needs to be a margain bigger than his opponents, sure he'd not seem huge in comparison to Bowser, but that's because Bowser himself already pulls this off in Smash Bros. Bowser is made huge in the 3D Mario games to show his might, status and make him incredibly threatening to face. Ridley does the same in his source games. However, it still works for Bowser in Smash Bros, especially with his new pose, because he's still the biggest and acts the baddest out of all characters. Ridley can pull the same trick off, he just needs to be about the same height as Bowser while hunched over. And yes, it'd be hard to make it feel natural when Ridley is getting smacked about if his pose is constantly hunched over, but they did it with Bowser in Brawl and Melee, and unlike Bowser Ridley can also run on all fours, so he should actually have an easier time being hunced over than Bowser had.
As for Ridley in the air, it's a matter of scaling the proportions right. What you see most hacks fail at, and why he seems so small, is because they don't scale proportions. If Ridley has to have a smaller wingspan you don't just scale the whole model down, you just scale the wings down, and then maybe scale other body parts down a bit too to not make it look too unnatural. It's just about fine tuning and hitting the right spot in regards to proportions, which is another thing that's hard about implementing Ridley. It's a general rule in 3D to do so, and it happens regularly to Ridley in the Metroid games too in-between games and appearances.
No, there's no way to make him 100% faithful as a playable character, but it's easier to do that with him as a character than as a boss since Smash Bros bosses remain too stationary. Even then not all characters are faithful to their original counterparts, look at Ganondorf, Lucas, R.O.B, Captain Falcon and Ness, they all do different things that they can't do originally to compensate for different reasons (though Ganon is admittedly just hurt from the Melee days.)