Not a lot of people auto cross their Mustangs. My best guess is that they can't handle getting their rear end handed to them by Civics, Neons and Miatas. Don't let that discourage you though, those cars are in a higher class for a reason.
I highly reccomend it, it is a lot of fun. If you modify your Stang, don't go for power goodies, they won't improve your time much, if at all. First thing you'll want to do is try to solve the understeer problem. I won't try to tell you how to go about that, everybody has their own idea of what's best and it depends greatly on your budget and what class you want to race in. One thing I will warn against is lowering the car too far. Many people think that lower is better but that's not true. A 2 inch drop will probably reduce your ability to hold a corner rather than improve it unless you've spent a couple grand on a coil over kit and all the related hardware.
If you stay in the stock class, you'll race in F-Stock. In Super stock or Street Prepped, you'll be in the E class. Couldn't tell you about any of they other categories I've seen in American clubs because we don't use them in this part of Canada.
Make sure you read the rules about each category and what mods are allowed carefully. My gears alone moved me up to Street Prepped even though I only have 3 modification points, which would normally mean I'd be racing in ESS.
All that assumes you care about where you race. If you just want to go out and have a good time then mod away and race in whatever category you wind up in, just realize that if you want to be competitive you need to know the rules and what mods will be most effective given the limitations of each category.
Also keep in mind that you're going to need a ton of practise to start turning some good times. Just like drag racing, you'll improve over time and there's no substitute for being on the track.
Last thing, if you get hooked like I have, get a second set of wheels and some hard core tires. You can trash your street tires pretty quick in this sport.