Fun fact: This is the only game where you can’t play as every character from a franchise at once. For example, you can play as all 4 Pokémon fighters and both of the Donkey Kong fighters in brawl. However, there can only be four fighters in a match and there are 5 Mario characters. Thus, you can’t do this here. In Smash Ultimate, though there are 9 Mario fighters and only 8 slots, you can stack echo fighters, which clears up one spot, making it possible with all franchises.
I mean, technically that works with Ultimate, the way you explained it, but I’d say that both Melee and Ultimate have too many Mario characters *cough cough* Dr. Mario *cough cough*