As long as it's un-plasticized PVC you should be fine. You may to look into weather green PVC has any plasticizers in it (I doubt it, but don't know for sure).
The media guard doesn't need to be 6". If you are building a timed flood and drain system with a stand pipe (no bell siphon), 4" works just fine.
Right. It costs manufacturers money to be able to make claims like that (even when the product is perfectly safe). The green colored sewer pvc is manufactured to ASTM (American Society for Testing and Materials...now just called ASTM International) D3034 specifications...It has more to do with wall thickness, since that type of pipe is meant for non-pressure, gravity fed (like drainage and sewer) applications as opposed to any sort of chemical make up of that product.
Again, as long as the PVC is rigid and does not contain plasticizers (to make it soft and pliable...you're good to go. If you or the hardware store guy are un-sure or can't tell...then yeah, your better off getting the white schedule 40 pipe.
I have used the green PVC in my bed with no problems. Never really thought about it before.