But the normal caveat applies - if your supply isn't able to supply the motor's max current requirements the Arduino will likely crash. Also you want to be sure there are no high voltage spikes on the supply that might overload the Arduino's regulator.
As for PWM frequency I don't think there's a specific library but it can be done by reprogramming the relevant timer module directly. If you want millis() and micros() to still work don't modify timer0.