Hi,
1. This is really a matter of what works best for your project visually. The default is 100 pixels per unit.
2. For all attacks, special abilities and reaction animation use the keys 1 to 9 on your keyboard. To Coruch hold down the C key. To walk instead of run, hold down the Left Ctrl key.
3. No plans to include projectiles in this character creator.
4. It sounds like something that you could set up yes. Perhaps just changing the tint/color on the sprite renderer would be enough and saving the value to load in everything you start the scene with that character.
Hope that helps!