1-Yes, projects do become corrupted... never happened to me, but doing a "save as" to a new project "every now and then" may be a good idea
2-I only use one kind of video in a project, so have never used different sequences... just one sequence and one timeline
Since you are new to PPro, the tutorial list in message #3 http://forums.adobe.com/message/2276578 might be a place to start