whoever told you that was a jackass.
the only way to damage the alternator is to overwork it and since electricity isn't a mystery anymore you can avoid this.
without the numbers there is no way of knowing whether or not your alternator can handle the load.
for example if you have some little econobox with a 40 amp alternator then you can't put a 2000 rms watt amp in it and be fine since a 2000 watt amp can draw around 140 amps on top of the power the car itself needs to run normally.