Actually it has nothing to do with gameshark, the ability to play as Sephiroth was a mod for the computer, which all it really did was replace Cloud with Sephiroth. Same with the Chocobo, goblin's, ect. These people were cool enough atleast to create a limit break that was different than Cloud's 4th level one. Other then that, all the limits are the same. And a custom sprite for Sephy running around the map. There was a similar Mod that also allowed you to travel the world as Cid or Tifa or Cloud, but none of the others.

There was a gameshark code however that allowed you to keep Aerith after she was killed, but she wont show up in any of the group gatherings to speak with. But you retain her skills and weapons.