What are they all, what Pokémon they are and how you unlock them.
There are 20 expert stages. Each stage needs a certain amount of S-ranks for each. The lowest starts at 12 S-ranks for Absol to 150 S-ranks for Mewtwo. You obtain S-ranks based on how you beat Pokemon on the main, special, and expert stages.
Check this list for all of them. Scroll down until "Expert Stages":
Hope I helped! :)