This answer is indented to be canonical answer for similar questions and applies to all devices that utilize USB (Type A or C) chargers and conform to the USB Power Delivery specification (version 2.x through 3.x).
If there is only the MacBook 12 inch USB-C charger nearby, can it be used to charge other USB-C devices, such as any Android phones that also use USB-C? (will it "too powerful" and damage other devices?)
Yes, it can be used and you will not cause any damage.
USB-C conforms to the USB 3.1 specification. Part of that specification is the Power Delivery spec. This means that the device and charger will negotiate the power that is delivered.
As to whether a charger is too powerful or not, you can always use a charger greater than or equal to the maximum power draw of the device. This applies to all chargers (USB, barrel connector, etc.), not just those that conform to the USB 3.x specification.
In other words, if your Apple USB-C charger is capable of delivering 87W of power, but you only need 10W for your phone, you can charge your phone no problem.
However, your 10W phone charger should not be used to charge your 87W MacBook. Why?
- If it's USB 3.x (USB Type C), it will negotiate power. Since it technically doesn't have enough, it will either charge slowly or not at all
- If it's USB 2.x (USB Type A) or a regular type charger, it will either not charge or have the possibility of damaging the charger because the draw of the device will exceed the charger's ability.
(MagSafe power adapters have a chip built into it prevent using an under-powered charger).
Vice versa, if there is a Quick Charge 2.0 or 3.0 USB-C charger from other devices nearby, can it be used to charge the Macbook 12 inch?
Possibly. The power delivery spec allows for power negotiation. If there is sufficient power, it will charge, albeit slowly. It may charge, but not power your device. It may do nothing (it couldn't agree on a negotiated power delivery so it stopped delivering power). If it's a poor quality charger (and does not conform to USB 3.0+ spec) it may overheat.
This is why you always use chargers greater than or equal to your device load.