How to replace a SMD capacitor on a raspberry pi?
As you wrote, this is a decoupling capacitor. When the current demand of a device increases suddenly, the power supply circuit may not be able to fulfil it immediately, e.g. due to the inductance of the traces.

Decoupling capacitors just jump in, until the higher current is delivered by the supply.

But real capacitors are not ideal capacitors. They also have a resistance and inductance. This may cause similar problems as for the supply. As larger capacitors have a higher inductance, the solution is to use a large and a small capacitor. The smaller reacts faster, the larger endures longer. 100nF and 10nF are very typical values.

But it's not absolutely necessary that you solder a new cap of exactly that value, 8nF, 22nF or 33nF would do the job, too, (also remember, many caps have a precision of just 10-20%). Also, it's possible that you don't need that cap to run the Pi. 

If you only have 1mm solder, just put some (not too much) solder on the pads of the PCB, add some flux, hold the cap onto the pads with tweezers, and solder it with a fine tip, without additional solder. 

If the cap is tiny, there is a chance that you produced a short circuit. You should test the resistance, it should not be almost zero. It's also possible that there is no electrical connection, but as said, it's possible you will not notice that.

But also note that soldering is thermal and mechanical stress for tiny components, especially when removing them. This may also damage your robbed caps, I woud always recommend to use fresh components.