Look for "Open Halter" classes. Whenever you see the word "Open" it means that it is open to all breeds, even grade horses. Chances are there will be at least one open halter class. I'll warn you now though that those classes are usually HUGE and the competition will be steep because every horse owner and their mother will be sure to enter. Just go with the goal of having fun.
Triple Crown Girl, a "grade" horse is a horse without a registered pedigree. Usually they are mixed breeds, but sometimes they are simply horses whose pedigrees are unknown. It does not mean they are lesser quality (don't let anyone tell you that), I have known some great grade horses that did very well in shows. All it means is that they will never be able to compete in breed shows. The cool thing about grade horses is that they usually sell for less money and so if you are not interested in doing breed shows you can sometimes get really good deals on great horses. It's not so good if you want to sell a grade horse, unless you have done a lot of showing with it. Once a horse has racked up a bunch of ribbons, no one cares if they are grade or not ;)
I hope things go well at the show for you and your Paso Fino!