There are a few things to know when dealing with used tuners.
1. If the car it was previously used to tune was never returned to stock, the tuner will be locked out and can only be used on the original vehicle. You would have to pay SCT to unlock it.
2. Since it's an older model tuner, it may need to have it's software updated to be used on a 2012. You can do this yourself pretty easily.
As long as the tuner is unlocked and the software is up-to-date, you should have no problem loading a custom tune to it and then tuning your mustang. One of our AM guys on here should be able to help you out in more detail.
2012 Kona Blue 3.7L V6:
Borla LT headers, O/R h-pipe, Borla S-types, 3.73 gears,
Steeda adjustable UCA w/ mount, BMR LCAs, Airaid CAI,
LCA Relocation brackets, AE driveshaft, GT 80mm TB,
Black 18x9/10 2010 GT500 Wheels w/ Nitto NT555s,
Eibach Sportline Springs/shocks & Koni STR.T Struts