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I assume you're talking about the sound reactive mod. You can really use any size LED that will fit in the space where you will mount the LED. everyone uses 0603 LEDs because they are so small and can be mounted in very small spaces. The 3528 size you have is approximately 3mm x 3mm x 2mm compared to the 0603 which is about 1/3 to 1/4 that size. I'm not sure how much space is available in the recommended mounting area for that mod, but the 0603 would take up far less space (and most likely use less power) though it will be slightly more difficult to solder. The choice is yours honestly. The only way you'll know is to test fit the LEDs that you have...
I don't know honestly... I will tell you that if you order from mouser.com you can order as few as 1 or as many as they have in stock (the more you buy the cheaper they are, bulk discount). they also have very fast shipping. I usually receive mouser orders within 3 days... check out some online parts suppliers before you order anything from a local retailer...