It works on 16 characters.
With the shortened footstool, it works on:
Yoshi, Bowser Jr., Wario, ZSS, Palutena, Marth (if you move slightly to the direction where he's facing while performing DAir), Pikachu, Charizard, Villager, Megaman, Sonic, Mewtwo, Ryu, Corrin, Bayonetta and Mii Brawler.
And the full footstool only works on Mewtwo.
Also, note that the sweetspot only works on Yoshi and Mewtwo, with the shortened footstool. The full footstool will perform the sourspot on Mewtwo.
I'm currently working on a video about that and all the combos that are possible.
EDIT: I just saw that you mentionned Dr. Mario, Falco, Luigi, Mario, Rosalina and Roy. It's really hard to get it consistently on them since you have to move a bit to a side.