Extra options:
Complete the game in story mode. Press Start at the title screen to display an "Extra Options" selection that allows story mode level selection and dream mode for the two player game.

Select opponent:
Press Start + 1 + 2 to select your player. This will allow you to select your opponent.

Special moves:

Press Down/Right + 1, Down/Right + 2, or Down/Right + 1 + 2 using any character.
Press Forward, Down, Forward, 1, 2 using any character.