So in my hack, I want to replace the models for Mario, Luigi, and maybe some other models, but I cant find any tutorials on how to do so. Can anyone tell me how to replace Mario, Luigi, or any models in the game?

As I mentioned before, replacing Mario's model isn't really documented as it's barely been done. This post I made lists some tools that may be of use, so maybe check that out.

It's nearly impossible to replace the models of Mario and Luigi.
But to replace other models, u will need to extract the nsbmd file.
Then convert the files to dae with apicula.
Import them into Blender or other similiar programs.
Edit the model. Export it as dae. Convert it to nsbmd with TheGameratorT's tool.
And then replace the nsbmd in nsmb5 with your edited model.
(note: TheGameratorT's tool doesn't support bones)

I doubt you need to edit Mario's model to replace it, but you should probably use it as some sort of height reference. Also, gamerator's tool wouldn't even work in this case as I don't believe it supports merging nsbmds. Best bet would probably be making the boned models, converting them to imd with ass2imd (assuming it supports bones, dunno off the top of my head), then convert them into a merged nsbmd with g3dcvtr.