Well shows how much I know. I never even knew about the extra costumes. How do you get them anyway. I take its by completing a certant part of the game.

To get a costume, just beat every mission with that character. To get the secret character, just get all A's with him.

Sonic = HUmar Sonic = Amy
Tails = Red Tails = Chao
Knuckles = Leather Knux = Tikal

Shadow = HUcast Shadow = Mecha Sonic
Eggman = Camo Eggman = Gerald
Rouge = Bat Girl = Chaos

I have no clue how to read the way you listed that.

Ok, here's a better list:

Character :: Extra Costume :: Secret Character
--------- -- ------------- -- ----------------
Sonic :: HUmar Sonic :: Amy
Tails :: Red Tails :: Chao Tank
Knuckles :: Leather Knux :: Tikal (SA1)

Shadow :: HUcast Shadow :: Mecha-Sonic
Eggman :: Camo Eggman :: Dr Gerald
Rouge :: Bat Girl :: Chaos 0 (SA1)

To get the Costumes: Beat all the missions with the corresponding character.

To get the Secret Characters: Get A Ranks on all the missions with the corresponding character.

I hope that's better.