First and foremost, the issue of size. You may argue that Sakurai resized Kirby and Olimar, but an increase in size is not the same as an decrease in size. Besides, their size change did not effect others that were in their rspective series, as the size ratio was still the same between the other characters of the series. Example, Kirby was resized, but Meta Knight was as well, so their proportions are the same. But if you resize Ridley, he will be too small in comparison to others in his series. Namely, Samus. Now, if they could find a way to resize Ridley without changing his proportions to Samus, I'd say PUT HIM IN. But, that can't happen without also scaling down Samus.
Second, most of his attacks are based on being in the air. How often do you see Ridley fight on the ground? It's happened, but not enough to have sufficient data on him. This, of course can be countered by giving him smash-unique moves, like Zamus. Regardless, he is mainly air-based and the air is not the best place to be in smash.
I think a seperate mode where you could play as bosses only would be good, however. And a third mode where you can play as either, if you're up for a challenge.