Re: DIY Rack... Whoo-Hoo!!!
Dado cuts are fine for PVC. I use it on all of mine. Here's the rub though. There's a fine line between getting it right, and having a melted mess on your hands. Aside from that, it's doable and makes for a much stronger design. You can do it with just the screws, but remember that the PVC is a softer material. It will benefit from the joinery and it adds a cool factor, and makes it a lot easier to assemble. Granted I am using a CNC table, but a router will get it done IF you use a guide. That's a must. AND remember that the PVC cuts like butter. Get it right on a piece of wood first. Also, don't make the span of the shelves too long or wide without support. It will sag if you are not careful, even the 12mm or 1/2 inch stuff. BTW, use the 6mm or 1/4 for the backing and save some $$$. It's not structural.
EDIT TO ADD. Don't use the 6mm for anything else besides the backing. It's too thin. The OP is right. Use the 12mm for the strength.
Dave