File compatibility issues when uninstalling Mountain Lion and installing Snow Leopard

I intend to get rid of Mountain Lion on my iMac and reinstall Snow Leopard. There will be a full backup of all my files.

My question is about the data files for iTunes, Mail, Contacts, Calendar, Bento, Word and Excel -- will the data files created or modified while Mountain Lion was the installed OS still be useable when Snow Leopard becomes the installed OS? Is there backward compatibility? Or do I need to export the data files? Or something else that needs to be done?

I want to keep iCloud only for email purposes. What needs to be done to make sure NOTHING ELSE on my iMac is accessed by or accessible to iCloud?

Thanks.