This project is very interesting ! I love 64 and PM, so its pretty cool to see someone make this project a reality (badum-tssss), with P:M, it's notable that the project is "Melee-inspired" and not Melee 1:1, so is this meant to be 64 with brawl roster or "64 inspired" (either one would be amazing btw im just bringing this up out of curiosity). If its the latter, I think you could definitely take aspects from other smash games or your own personal ideas, and apply them here. An example being side-b's, which honestly, could be seen as improvement over original 64, and makes balancing easier and makes Olimar possible to be added without being completely different lol. Just some food for thought I guess, but I appreciate the project no matter where you take it
Also to go back to the side b's and other aspects from other smash games, when you think about what PM is, it takes the best from melee and brawl and puts it into a melee paced game. The thing that really made that work was that brawl and melee were both complicated but different games, each one with there own style and techs, which left them with so much to flesh out and differentiate characters ( look at Lucas for example!). 64 on the other hand, while a competitive game, is much more direct and doesnt have as many advanced techs or mechanics that other iterations had ( not saying this is a bad design, thats just how it was). Now without these mechanics, its going to be much more difficult to keep the cast fresh, but this is where your own input comes in.
1. Dont stick to keeping the og 12 1:1 with the 64 counterparts, tone down what needs to be toned down, bring in important move changes to characters if needed, an example of a difficulty would be the space animals, you may have to switch Fox over to Melee lasers when you bring Falco in, to keep them unique and switching Falco's lasers to be like Melee Fox's doesn't really fit with Falco's style of play or kit
2. Keep melee and brawl vets somewhat faithful to how they play in their best iteration, but balanced out for 64, exceptions being if it doesnt fit with 64's style (cough Lucario), it'll make the game more accessible to those who want to try out the game with someone familiar.
3. Be original when needed, but dont go overboard, dont lose sight of what the project is supposed to be. Just always remember what comes to mind when you think of Smash 64 and just dont mess with the core idea behind it
Im sure this can be a fantastic project with a little work, but dont be afraid to not make a 1;1 reproduction of smash 64 and if you think my ideas are stupid and dont want to implement them..... well its your project not mine, just wanted to give input and i look forward to playing it in the future
TL;DR: read it, its important;]