The problem with Mustang wheels is, well, they're made to fit a Mustang. First off, 14" wheels probably don't fit over 05+ brakes. Even if they did, Mustang wheels are usually around et10 offset, which looks pretty silly without being really low and a good amount of camber adjustment.
The only Mustang wheels I've seen done well on a Focus are Cobra replicas (17x9 et10, I believe) on a slammed Focus. Even then, the poke is a little ridiculous.