Okay, in order:

The measurements for the slide cards, both designs, are wrong. Mine ended up being just a smidge shorter than a credit card in length, with the width in the same ratio. It doesn't need to be exact.

Yes, that is CDLoader. However, you're more likely to find a newer image at http://www.ps2ownz.com

The reason why PS2Reality might not be playing your MP3s is because it falls outside of the boundries of your boot disc, assuming you use one. What is your boot method?