some alternators wont operate at idle, but with a few revs the red light goes out. The higher the output (of the alternator) the more likely this happens. I have several cars that the light is on at idle, but still charge when driven. If this is the case, Im sure it is no worry, but if you change the pulley you may make it better, probably a smaller pulley to make the alternator spin faster.
Just a suggestion only, speak to your supplier first

cheers
Max