About clay pellets: Yaaay. Good choice. I've had nothing but good luck with these. Totally reusable too, so they are free after the first purchase.
About Co2: Make darn sure you plan this into your setup and don't think you can just add it later. You will have to tear everything apart. I didn't really understand everything you said about your ventilation system, and maybe you got it covered. A system with a good co2 settup must be planned very carefully. For example, you only release co2 when lights are on and there is no ventilation. Lights on though, makes a lot of heat. You must have a way of removing that heat without removing the air in the grow chamber while co2 is on. You'll want a cool tube or two, but a sealed hood will work. The air within the grow room must be circulated, and it must be replaced many times throughout the day. Don't forget that everything has to be sealed tight, unlike rooms without co2. You said you want a basic ventilation system, but I don't think that's possible with co2, at least to do it properly. It's too bad that you should plan carefully for it because I would suggest not using it until you get everything else dialed in just right. Don't forget the big, huge fans, and lots of them.

I hope that doesn't frighten you. I have all my co2 parts sitting on the growroom floor. Exactly the same place they've been for the past year.