It is no trival task to set the clock fuses correctly on an unfamiliar PIC. I suggest that you set your fuses for the required rate, then write a program that flashes a LED at a known rate (eg 1 per sec), then ensure that reality agrees by manuallly timing by conting flashes.

Your enquiry re. low speed piqued my interest. Having never used low speed I did a little googling and came up with some contradictory "answers". See http://www.compuphase.com/electronics/usb_lowspeed.htm and http://www.rosseeld.be/DRO/PIC/18F24..._Usb_Clock.htm