In adult onset primary hypersomnia, is it reasonable to take beta blockers if effective in contolling the high pulse side effect of methylphenidate?

Not exactly. Beta-blockers are helpful for a variety of medical disorders, but like many medications are not entirely benign. Side effects include daytime fatigue and sleep disruption. What you have described is not a clear indication for beta blocker therapy - risk may outweigh benefit. Please consult with a local sleep specialist for further information.
Hypersomnia. Yes. The beta-blockers should work in this context; they will slow the pulse, but you are also getting a new set of potential side effects, one of which is drowsiness. It is likely that the methylphenidate will outweigh this effect, but it is an individual matter. You'll have to try it and see. Otherwise, it is quite safe.
Somnolence. Speak with the doctor(s) who prescribed the two drugs; tell them your current symptoms to see if they might recommend a change in the dosages of the medications.