Nesquik is mostly sugar. According to the nutrition info, 13g of each 16g serving is sugar (that's 81%). Most of the other ingredients you don't need in beer either, but nothing that will hurt fermentation. The sugar means you'll have to add at least 5 times more nesquik by weight to get the same chocolate flavor. Also, the chocolate flavor is from - cocoa powder. So I'd stick with the unsweetened cocoa powder, which is entirely cocoa.
You can add cocoa powder at the end of the boil. It will turn into sludge in the bottom of your kettle. Bad news if you use a counterflow chiller or have any sort of screen in the kettle.
Are you adding any coffee? I'd add 1 or 2 ounces of whole coffee beans after primary fermentation for subtle coffee flavor.