Let me preface this post with "talk about this with your doctor."
Of course you know people can respond differently to drugs. Try to find something that can replicate the situation, like a toastmasters meeting where you have to give a public presentation. Might not be ideal, but it might approximate the anxiety you'll feel at an interview.
If you don't feel anxious at the toastmasters meeting then don't bother.
If you do, test it out see how you do. If it doesn't do the trick, talk with your doctor about adding xanax and/or increasing the propanolol. Rinse, recycle, and repeat.
If you can't replicate the anxiety at your toastmasters meeting, schedule the first interview you get with a school you don't care as much about. Try the medicine during that interview, if it goes good, try it out for subsequent interviews.
Even though I wrote these statements as directions, I want to say that I am actually phrasing all this as just hypothetical suggestions and this does not constitute medical advice at all. I'm no professional, I'm a dumb premed who just writes ideas. Talk with your doctor always so they can take into consideration all your relevant health information before making a suggestion.
Thank you.