Can I plug my 15" 2017 MacBook Pro into a USB-C display while using the charger cable?
It shouldn't be a problem.  I did the same thing with an LG 5K display, and the power plugged in separately, and had no issues.

Having written that, I almost never bothered to do it that way, because my laptop spent most of it's time on my desk, and I never needed to charge it faster.