To unblock Metal Sonic you must end the game with the other characters: Sonic, Knuckles, and Silver Shadow.
To unblock the skin on each of the characters, it is sufficient to complete the adventure with him. For example, to have the skin Knuckles, it is necessary to complete the game with Knuckles.
To unblock further cuts Cup Circuit mode, you must complete all the challenges of each area in Easy mode, Normal and Hard.
Challenges to complete Cup
Blue Chaos Yard Death
Cyan Colosseum Highway Chaos
Chaos Green Forest Falls
Chaos Gray All Zones
Purple Chaos Meteor Base
Red Sky Park Chaos
Yellow Chaos Crystal Mountain