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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 be "too powerful" and damage other devices?)
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?
The main concern is about whether it could damage the device or battery.
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 be "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 with no problem.
However, your 10W phone charger should not be used to charge your 87W MacBook. Why?
(MagSafe power adapters have a chip built into it to prevent using an underpowered 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.
Short answer
To address your main concern about whether it could damage the device or battery when charging devices with a charger not specifically designed for that device, the answer is yes, it could. However, in some cases it will be fine. The only way anyone here may be able to answer that question is to know the exact models of the device(s) and charger(s) you're talking about.
Long answer
While I understand your question, it's impossible to give a generic answer.
The reason for this is different chargers will have different voltage/wattage ratings.
Using MacBooks as an example, they typically require in the range of 55 watts to 90 watts of power in order to charge (depending on the model, e.g. the late-2016 13" MacBook Pros require 60 watts, while the 15" models require 85 watts).
So, it is possible that you have a USB-C charger from another device laying around that provides enough juice to charge one model of MacBook and not enough to charge another. Or, it may be enough to charge one model in a few hours but not enough to charge the other model unless it is switched off and connected for a day.
Another important factor is that different devices will regulate how they draw power for charging differently. One way people often get caught out is that they will use a charger to charge a device that draws power more quickly than the charger was really designed to do. This seems to work fine, but after a while what happens is the charger itself dies because it's been used too often to charge that particular device.
Another way people get caught out is they connect a charger which has the wrong voltage, because it is close to the correct voltage. However, if the device is using that charger as it's main power source the risk of damaging the device is pretty high. It's just not worth the risk!
NOTES:
The macbook does not support quick charge 2.0/3.0. It supports USB PD (Power Delivery). With USB PD, both the charger and the charged device negotiate a voltage and amperage. If the charger can't supply a power profile amiable to the charged device, then charging won't occur. If you charge a USB PD device with a USB PD charger, then you aren't going to hurt the device or the charger. The macbook power supply supports USB PD and can be used to charge other USB PD compatible devices. I have charged my google pixel with my 2016 macbook pro power supply (87w).
I have a 15" 2016 mbp with usb-c. I've charged it with my Pixel charger (15w) just fine. Under light use, I can use my laptop and charge the laptop at the same time.
Short answer, I tried it with my Google Pixel C and it works fine. Also, check out: http://thewirecutter.com/reviews/best-usb-c-macbook-and-laptop-chargers/. Resist the FUD.
In general you should be fine. In cases if it is not going to work, you will immediately know when you check the battery status in settings of your Android phone. It will usually say not charging. As other answers pointed out, if the device depends on QC 2 or 3 (marketed in different terms such as Motorola torboppwer) I found it not charging from MacBook Pro charger. I tried following phones with my macbook pro USB c charger Samsung Galaxy note 8 - works Google pixel 3 - works Moto g6 - doesn't work
Ok so, my macbook charger is bent so I have to use an android charger.
Basically its fine, but it doesn't charge quickly, and the battery can go down and up when using the charger. It works best when the macbook is off, so I'd say if you are using one for your macbook, charge it for a few hours at a time while its off. (But getting an actual mac charger is best!)
