Well to respond to this rumor you have heard about "being only able to play as Lightning throughout the whole game" is false. Throughout the early chapters of the story "you", the player, will be in control of every character at some point as the game frequently changes. Sometimes Lightning is in the party but is not the lead/playable character. Now, who is in your party is not up to the player as it is predetermined by story and until the player reaches Pulse; in Pulse the player is allowed to put any character into their team.