Well, about the led's, they both carry the same voltage information, the only difference is the size. The 1206 led, is a tiny bit larger in size than the 0603. For your modding project, I would recommend using the 0603 size led, simply because of the limited space that the leds will have once they are install. If you feel that the leds are not bright enough once they are installed, then your next step is to bridge the resistors. This will not harm the leds.