Downgrading from OS X Mountain Lion to Snow Leopard gets stuck at Apple logo
The reason you can't boot from from the USB stick may be because it's a version of Snow Leopard that predates the one your Mac shipped with. Is the source version of Snow Leopard you're using a retail disc?

Also, downgrading directly from a newer OS is not supported behaviour. I've seen it attempted successfully (at least superficially) on a few occasions and, among other things, the OS does downgrade but the data doesn't. So iCal's data is geared up for the newer OS, etc etc.

So. The only way you're going to be able to do this properly is going to be by wiping your internal hard drive with Disk Utility, installing Snow Leopard, then setting everything up from scratch (manually installing your apps, moving important data, etc). I know it sucks, but it's going to be the only way everything will work properly.