Ridley is a dragon, but if he is the same size as the other characters, then he still probably wouldn't weigh all that much. They're gonna have to shrink him a little to put him in, but it could defnitely work. He was already good in the opening movie.
Meta-Ridley is cooler, my thing is how realistic is it? If he's half robot, he'd be heavier, stronger, and have more weapons (similar to Ghidora and Mecha-Ghidora). If that was the case, it wouldn't be realistic to me for him to be a normal character, it would make more sense to me if he were a boss character (18 murderous rampages). That's why I think the old, non-meta Ridley should be used, so it's more believable. In the end, it doesn't matter as long as he's in. I wouldn't care all that much if he had alternate costumes as Meta or even if he was altogether.
After looking at your pic, Marshigio, that Ridley does look better. But the Meta-Ridley I'm thinking of from Metroid Prime was more metalic than that, he looked like he was half robot to me. The one from your pic would be good, though.