Can I charge the MacBook Pro 13" with the 29W adapter?
As with any computer, using a higher-wattage power adapter is fine, but using a lower-wattage one might not let it run at full efficiency and may be unhealthy for the battery in the long term. (And of course it won't charge as fast.)
There are a number of answers to this same question, but for MagSafe chargers, here. Kuba Ober's answer goes into quite a bit of detail about it, though I can't personally vouch for its accuracy.
Theoretically you can use any USB charger to charge your 12" Macbook, 2016 or newer 13" & 15" Macbook Pro.
By using very low power chargers like an iPhone charger your charging time will increase by many hours.
Using more powerful chargers will give the same result as using a standard charger. There are some exceptions however:
- iPad Pro models (2016 & 2017 at least) charge faster with the 29W 12" Macbook charger. 61W or 87W Macbook Pro adapters also work but have no benefits over the 29W charger for the iPad Pro models.
- iPhone 6s, 6s plus and newer iPhones charge faster with a 12W iPad adapter. This is also true with 29W, 61W or 87W Macbook (Pro) adapters although they won't charge faster than with the iPad charger.
There is no proof that using a quality original charger & cables will pose any harm to yourself or your devices when using USB power adapters intended for different models. Devices only pull the power they need but may refuse to work(charge) when they can't get enough. Using a charger that is way too powerful for the device may result in higher overall power consumption (from the grid) however.