On Shot shell reloading, generally you get one press and separate dies for each gauge. 410 and 28 gauge are uncommon. 410 reloading used to be considered difficult and expensive to load, can't really say for now.You have to consider the expense of the wads as well as the powder and shot.
Back in the day, Remington and Winchester shells used different size primers, don't know about today. Federal used Winchester size primers.
Mec(originally Maryville Engineering Co.) makes one of the more common loaders. You can probably look them up on the internet for details.
If you want 20 gauge empties I can probably send you a few, I've been after squirrels that have been after my pecans, an I don't reload 20s.
BTW, you can make a waterproof match case by trimming a 12 and 16 gauge case and fit the 16 inside the 12.