Are Android chargers universal in terms of power (Voltage, amperage)?
Some devices use USBMicro, others use USB Mini. Like any device, it depends on the voltage that the device can take. But for the most part, AFAIK, as long as the charger is the same plug type (micro, mini, etc.) then yes it can work. 

A lot of the chargers that use USB plugs output the voltage that a standard USB port would (between 4.75 and 5.25 volts). If this is the case, you will have no problems. The only issue, when you use USB to charge, it is usually a slower charge then using a wall charger.