mode field

Yes, 100 MFD and 80 MFD capacitors will work in parallel, assuming they have the same voltage rating. The net capacitance will be 180 MFD.

Yes, we can replace it

Field diameter of lens B equals field diameter of lens A times total magnification of lens A divided by total magnification of lens B

Sometimes. The exception is when the capacitor is used as a "timing" device. But 7.5 mfd and 5 mfd are so large that one would guess that they are not being used as timing devices.