🙂
My rules of thumb:
If the pedal is labelled 9V then don't plug in a 12V supply or it might be damaged.
If the pedal is labelled 1A and you plug in a 300mA (ie. 0.3A) power supply then it might not work, or the power supply could trip (if it has protection), or the power supply could be damaged.
In summary: a. make sure the voltages match, and b. make sure the PSU can supply at least the required current.
Other things to be aware of:
Some PSUs have discrete outputs, meaning they can be combined to give higher voltages or currents, but some do not and if you try and combine outputs of those, they will be damaged.
Most pedals have the Boss standard power: 9V DC 300mA, centre -ve, but not all do, so be sure to check. (Incidentally, centre -ve was a bizarre choice; most electrical equipment uses centre +ve, so I've no idea why they did what they did.)