OK, I got a gauge earring about 2 months ago to put in my cartilage. I got a size 18... and I was wondering... would it be dangerous or stupid to go from an 18 to like a 15? Or a 12? All that could happen is an infection, right?

A 12 is pushing it. If it were me, I really wouldn't go bigger than a 14 or 12 in my cartilage. You definitely can go bigger than an 18, but don't go too big.

Your cartilage can take up to a year to heal fully. It is best to wait until it is fully healed before you try to gauge it. The slightest irritation can cause your cartilage piercing to get "angry", trust me. I went swimming after having the piercing for a month and my industrial got irritated, so I would wait before you gauge it.