I noticed on some of these aftermarket brake booster and master cylinder setups they say the m/c can be used for either disc to disc or disc to drum setups. I always thought the m/c that are used for disc to disc are different. Or am I missing something?

Most master cylinders for drum brakes have either 2 smaller fliud reserviors, or 1 big (typically for disc brake front) and 1 small.

Wheel cylinders don't need as much fluid to activate them as what a caliper needs.

Aftermarket master cylinders advertised for both are really just disc/disc designed master cylinder. If you have drums on the rear, you just have a bigger fliud reservior.