I haven't taken xanax the same time as taking percocet but I have taken clonazepam (same family of drugs as xanax) as well as flexeril (muscle relaxant) and have been fine. Since you are new to combining these meds the biggest thing to look out for is drowsiness so don't drive if you don't have to until you know how the two will affect you.
If your concerned your doctor or pharmacist are there to answer you questions. They may tell you it's fine but also go by what your body is telling you, if the two make you sleepy take it easy (this side effect passes) or call them and ask if what you are feeling is normal. Don't ever be afraid to call your doc, they are hired by you to help you so don't think you are bothering them.
Good luck, hope you feel better soon.
Susie
P.S. I've had panic attacks, they suck, bottem line, but it does get better I didn't think it would when I had them on a daily basis but I made it out on the other side and so will you.