Topic category: Mod ideas exchange
So, after doing an Hour of Testing, I have made a Procedure that Randomizes Skins.
Here's the code in Screenshot form you can do the rest yourself.
It's compatible with Spawn Eggs btw.
You can configure the mob spawn chances to your liking.
This works best on 1.12.2 Generator for 2020.5
Edited Chances to 0.85 for this just to show it has full compatibility for more than just 10 or even 20 Mobs.