Is it dangerous to take stimulants (adhd medications) and beta-blockers (propranolol) together? if so, why? diff. med. for bp and migraine prevention?

Theoretically yes: Practically probably not. This answer is based on the fact that if you are on a B Blkr and taking a symptathomimetic agent like a stimulant this could cause severe hypertension. Based on studies of people on B Blkrs who took cocaine or were given epinephrine for allergic reactions and this turns out not to be unfounded and not the case. If your BP is up see your Dr and review your medications.

Yes, not advisable: to combine stimulants and beta blockers, may cause hypertension as they compete for the receptors controlling blood pressure, hold either, or seek a different treatment for either high blood pressure or migraine, ask your doctor, unless your doctor is aware and is monitoring you, best wishes