Entire books have been written on these subjects. It is a very in-depth subject, not simple.
But basically, when compliance gets lower, effective mass should get higher. Ideally, you should measure the exact combination for resonance in your exact system for best accuracy, but general formulas for mass/resonance matching are on the web. Acceptable combinations usually range from 9Hz-12Hz, although many go slightly outside that range and still work ok in many applications.
Cartridge loading for MC cartridges is related to the manufacturer's designed load for linear output of the cartridge. It should be followed reasonably closely. If you don't know what the loading should be, you can start at about 4-5 times the internal DC resistance of the cartridge windings(into an active phono stage). For most step-up transformers, you load at the tranny at about 1/2 of what you would load at the active phono stage(and leave the active phono input at standard 47k ohm).
Gain matching relates to the output level of the cartridge, and the gain ratings of the phono stage that are needed to bring the output of the cartridge up to a useful line-level signal. Lower cartridge outputs require higher phono gain settings. Since the gain in your preamp and amp will be influential in your overall system gain, they should also be figured into the equation(especially linestage gain), although you'll need as much gain up front in the phono section as possible, to keep the noise floor low.
Each system responds differently because different components have different gains. However, some generalized rules can apply. Most MM phono stages have somewhere around 40-45db gain, and most MC phono stages have somewhere around 60-65db gain. The only time you may need more than that, is with a very low output MC that is below 0.3Mv output. Adding the linestage gain(typically 10-20db) into this will bring you up to 75-85db, which would be fine for most anything from 0.3mv output and up. It is useful to have a range of settings, because if you use 85db gain on a higher output cartridge, you'll overload the preamp in most cases. You can "play" with the settings to see which sounds best, and gives you a useful range in the volume control.
