During the game, you will encounter Master Hand, who is the boss on Super Smash Brothers and one of the bosses on Super Smash Brothers Melee. After you kill him, he gives you Smash power, which gives you several abilities mixed up. However, these abilities are in the same control sequences and combinations as Kirby's controls for Super Smah Bros.
Boss Rush Mode is on the menu
Metaknight's Sword will appear in front of the Magic Mirror once you've unlocked it
The Power Center door is in the main 'hub' of Rainbow Route (Area 1) in the top left corner and contains all of the Powers in the game
The Sound Test/Kirby Colors can be accessed through the Treasure List