So with my modpack, the game will crash before loading the battle, seemingly dependent on the number of players. For example, on my personal Wii U (which has nothing else on it), the game will crash with any number of players above 6. However, on my cousin's Wii U, the game will crash with anything over 3 players. Explain?
It's entirely possible that you are trying to load a fight with certain mods. Some models are very high poly, so crashes tend to happen when the game tries to load all of that information at once. Unless this happens even with default costumes, I suggest testing out 8 Player by yourself starting from Mario (or whoever you modded first from top-to-bottom, left-to-right), with the other CPUs being other modded characters. Go through their costumes one-by-one, and if any act funky, just work your way in isolating who the troublemakers are and just set a rule that soft-bans certain characters for 5+ Player Smash.