These are the main people who you could waveshine against.
Easiest : Link, Peach, Zelda
A lil Harder : Captain Falcon, Sheik, Bowser, Donkey Kong
Harder : Ganondorf, Samus, Mario, Doc Mario
I have prob got some in the wrong order and missed a few out because its off the top off my head so sorry if i have but i defintely know you can waveshine all of them in practice mode.
MAKE SUREEE when your waveshining though you don't let go of the stick you just tilt it forward which allows you to walk a little bit otherwise waveshining with people who slide futher won't work properly, and tilt it enough so you get the full effect of the waveshine but if you start off with people like CF and Sheik you should get the tilting of the waveshine easier as you would have to use it unlike with Link you don't have to tilt it. WALKING IS IMPORTANT BASICALLY lol
And no there isnt a percentage where you cant waveshine the waveshinables as the shine makes people slide the exact same distant even at 999%