I find I quite like the witch's new (to me) Arc skill, to my regret, however, I discovered that Lesser Multiple Projectiles does not work with it. Which support gems, especially of the projectile modifying sort, DO work with Arc?
The ones that I have used, and can confirm work with Arc, are
Blood Magic Faster Casting Lightning Penetration Reduced Mana Cost
List of support gems can be found here.
Any others may work, but I have not tested personally, therefore would be total speculation, on my part.
Arc is not a projectile, so Multiple Projectiles won't do anything. Chain does work though and causes it to chain to more enemies.