Alright brehs, there's a lot of silliness in this thread and I don't know how to formats posts very well.
Multiple Melee versions on 1 USB? YES! I have done this already.
The Game ID does NOT require any specific characters to be properly read in DIOS MIOS. (Myougi's post about GALE01 needing the 'E' is incorrect.)
To change the Game ID, you do NOT require a hex editor. Hex editing is tedious and much harder than using a simple program.
And DIOS MIOS does NOT need to put ISOs through DiscEx first!! DIOS MIOS is looking for this pattern in the file system: root (so *DRIVE LETTER HERE*)\Games (or games, capital letters don't mean anything to DM)\*GAMEID HERE* (and GAME ID doesn't matter either.)\game.iso
Game ID matters so little that the ID of the ISO doesn't even have to match the name of the folder!
DM doesn't care what the folder name is and it doesn't care if the folder name is the same as the Game ID of the ISO in question.
For example, I have D:\Games\GALE69\game.iso AND D:\Games\GALEXX\game.iso both being read as Super Smash Bros. Melee because DM only looks at the ISO's information. In short, as long as your ISO is a Dolphin-readable Gamecube game, DM can also read it.
The good and easy method of changing Game IDs and having things recognized in DM is this:
Get this from here:
https://gbatemp.net/threads/wii-iso-gametitle-renamer.145064/page-3#post-2242399
It says "Select Wii Image" but it actually works with GC games too. So make your ISO of whatever mod(s) you like, compile it using GCR or whatever you're using, then open it with this program. The displayed name in DIOS MIOS will be what's under "New Title:", changing the game's ID is useful for very little other than differentiating Dolphin screenshots.