Short Answer
Guitar pedal power supplies need to match the voltage of your pedals. The power supply's voltage cannot exceed what the pedal can take. Guitar pedal power supplies can exceed the amperage requirement of your pedals.
Long Answer
While it is true that there is some strict science to guitar pedals, it's truly not as hard as it might be cracked up to be. There are a few things to keep in mind while you set up your pedals and design your board, but don't let that dissuade you from setting up something unique. Voltage and amperage aren't really that intense to wrap your head around.
Voltage is the first thing to pay attention to when looking at pedal power. You're likely to see, above anything else, pedals requiring a nine-volt DC center negative power supply. This will be your most universal kind of power supply and having one will power the majority of pedals these days. For most boards, all you'll truly need is a nine-volt DC center negative power supply.
Amperage is where things get slightly more intricate. Amperage, measured in milliamps (mA), has a requirement on every pedal, also. Where voltage has to be matched with your power supply, amperage can be exceeded. In short, a pedal's mA requirement can be 50mA and you can use a power supply with an amperage output above 50mA, no problem. All you need to pay attention to is that your higher-end, more powerful pedals will likely have a higher amperage requirement, usually near 500mA. Watch out for guitar pedals that require a higher amperage to function as that can be an easy hangup when you pick up a new pedal!