You'll definitely need a Snow Leopard disk in order to boot the machine, wipe the hard drive and then reinstall SL. Give Apple customer service a call and order a Snow Leopard retail disk from them. My understanding is that they still have them, but you have to call.
As far as going back to SL, that's up to you. I'm currently using ML on this Mac but to be honest, I liked SL a lot better. But with Apple, if you don't move forward, you'll eventually be left behind.