Charging a Macbook pro 15-inch (late 2016 touch bar)
Officially according to Apple: 


  You can use a compatible higher wattage adapter without issue, but it
  won't make your computer charge faster or operate differently. Lower
  wattage adapters don't provide enough power.


There is no mention of risking damage to the battery, but you may not be able to charge at all at lower wattages.

Additionally, you have to use a USB-C cable that is meant for charging. The Switch cable appears to be made for such.

In personal experience with MagSafe-powered MacBook Pros, the battery indicator in the menu may say "Battery is not charging", but I've noticed it stays powered on longer than just running off the internal battery alone. Your miles may vary.

Source: https://support.apple.com/en-us/HT201700