What's CS? I've heard of a few people using licorice, but I've never heard anything bad about it, I'm really surprised. Ok, I'll definitely be sure to stay away from that.

Yeah, that rib cage size definitely doesn't get taken into account for bra sizings. That's a lot of the reason why they're always so wrong. :/ But your underbust is a pretty good measurement, and should you decide to corset train in the future, it can get even smaller. You are very lucky that it's so small to begin with. I've seen models who have an underbust of 30-32 inches, but still look very petite because of their small waist. So once your waist shrinks, you'll look VERY tiny! You sound very shapely as-is just judging by your current measurements.