Mini 360 buck converter: 5v to 3.6v possible?
The term you are looking for is dropout voltage, that is the voltage drop between the input and output of a regulator.

The Mini360 modules, like most other buck converters, have a very low dropout voltage and you are safe to use it. I have not found a datasheet of the specific chip used on this modules, but you can be certain that it will work fine from 5V to 3.6V.

I would avoid feeding 3.6V to a ESP8266, as that is its absolute maximum rated voltage is 3.6V, which means that any tiny glitch could blow it up. I would stay on the recommended 3.3V, and put a nice large capacitor to smooth things out.