You can use the 50,000 RPM's, but it would have to be attached to a gearbox so that many RPM's aren't getting to the prop. As suggested above, max RPM's on a prop would probably be safely around 13 to 14 grand, although lots of pylon racers see closer to 15 grand and slightly above with some of their tuned pipes and such.
I would either load it to around 10 to 13 grand, or gear it down to 10 to 13 grand.