In my opinion your better off using the new sim than taking a chance on cutting it down.
Hey, misogi! I have to say, I'm inclined to agree with gramps28 -- the idea of cutting down a SIM makes me nervous! But if you purchased this extra SIM starter kit in a store and it hasn't been activated I believe you should be able to bring it back, since it's such a recent purchase. Who are we to deny you the adventure of being a SIM surgeon?