I think there's a lot of detail on the other thread re: the oxycodone/percocet issue. Percocet is also a C2 narcotic, at least in the US, so I don't know what your pharmacist was referring to. It's the oxycodone (which is the narcotic in the percocet, combined with tylenol) that's the narcotic, regardless of whether it's on its own or combined.
I take percocet so I don't know if the oxycodone is any different. Medications are so invididual, anything's possible. Plus, if you're on it long term you probably need to reduce the amount of tylenol you take (like I'm probably going to be doing soon).
Breaththrough pain is just that - pain that breaks through your main pain management medication(s), for example if you take something designed to last 8 hours, but it doesn't quite last that long, or you have some pain that breaks through despite that drug, your doctor may order another medication - perhaps a different type, a shorter acting drug, or even an over the counter med - to help with the pain that breaks through. Because the stronger the pain gets, the more difficult it is to manage, and the more medication you need to manage it. So controlling breakthrough pain helps avoid high levels of pain - at least, in theory. Problem is it's all a delicate balance, and very individualized.
Hope this helps.