I recently got a MacBook with the USB-C charger. It comes with a 29-watt power brick, and I was wondering if I should use this to charge my iPhone. I've heard that the iPad power brick (I think 10 watts) charges the phone faster. I don't expect to be able to charge the phone proportionally faster, but would the MacBook brick damage the phone? I would rather only have to carry around one charging brick around campus. Also, I have a USB to USB-C converter.
Unless there is some reason to the contrary that I'm not aware of, yes, you can use your MacBook charger with your iPhone. The power adapter will only convert as much power as is drawn, and the iPhone will only draw as much power as it needs.
(Note that it is not a good idea to charge your MacBook with an iPhone or iPad charger!)
This video shows that you can use MacBook 12" 29W USB-C Power Adapter with iPad and iPhone. https://www.youtube.com/watch?v=fWzbaH24Q0Y
Yes, at least for the iPhone 6S using the Apple USB-C to Lighning 2m cable. I just started using it this week with mine and haven't had any issues.
For a few years I've charged my iPhone with an iPad charger. Not sure if the 29W charger is faster, but at least it seems just as fast.
Also, I heard in a podcast about videos warning about USB-C to USB converters. As long as you trust the one you have and it's not sketchy, I think it should be OK.
My Macbook Pro 13inch Late 2016 61W USB-C Power brick says output: 20.3V/3A, 9V/3A or 5.2V/2.4A. So yes you can charge your iphone with the Macbook charger. The people at the Apple store just don't know what they are selling.