The red CD is Shouso Strip (first press)... which seems to be more common than the second press. It's in every 'bargain bin' in Japan second hand stores from my experience haha.
For someone who's starting out buying, ZCS is only worth it if you don't have
Watashi to Houden, which features the second 2 discs in their entirety. Of course, you have to buy ZCS if you want the first disc. Most DVD drives with a tray should still be able to support the smaller discs btw. If you have a drive that 'sucks in' your disc ala ps3 style- then you're stuffed.