use one of the online 'LED calculator's, google that, do the calculation, and you should come up with the precise value for your particular LED. adjust upwards from there, just don't go below the value it gives you.
assume the voltage to be 5V coming off the arduino, 5.5V if you want to be a little safer. and, calculate it for the particular LEDs you bought, you should be able to look up the datasheet for those
its fine to do. you'll increase the overall current draw of the device, but very slightly. resistors reduce the flow of current, which also has the effect of lowering voltage. so lessening the resistance will adjust both those values as well (this is Ohm's law, fyi).
so, if you are using a console to power the arduinoboy (rather than USB or its own power supply), this is disadvantageous for you to do. the current draw difference is pretty minimal, but still, there is *some* difference.