The fpu can have its own clock and run at a different speed from the cpu...if the motherboard has support for that, which very few will have as that wasn't very common back in the day.
You can definitely underclock and run the 33mhz part at 25mhz.
I would try running the fpu at 25mhz before you do the oscillator and overclock the machine, just to make sure you have that part working.