The manual says that the clock is roughly 50kHz and that it is dependent on the oscillator speed but it doesn't say how much. I guess you have an opportunity here to do some investigation and documentation and try it at a couple of different speeds and see what it does. Let us know.

No, the two devices does not need to run at the same oscillators speed. But, obviously, if you're using two PIC chips where the master is running at 64MHz and the slave a 4MHz it's possibly (or likely) that the SPI Clock(being generatd by the master) is too fast for the slave to keep up - but it depends on how much impact the oscillator speed actually has, so back to question 1.

/Henrik.