Checked the preorder FAQ!
You'll get the preorder items, no matter which code you use. It is only the Windows CE-Item code that you can't add to an existing Windows-service account. The pre-order code(s) will work. If you use a PS3 CE code and type it in on your console, then you can choose if you wan't to add it to an existing FFXIV- service account.

P.S.: All your characters on the same service account will receive the pre-order items! ^^