Ah - I was wondering whether you would have a problem (reason for italicised "should"). I think its version 9.0.9 that is required but as far as I can tell there seems to be problems if one had not updated to v. 9.0.9 before upgrading the OS. I updated to v. 9.0.9 but it still does not launch under Yosemite - it just bounces endlessly in the dock though it does work if I boot into Snow Leopard. I had already switched to iMovie 10 (and now FCP X) so it was not a big deal in my case.
Having said that, most people report that it runs fine under Yosemite which is why I responded how it did. Maybe someone else can chime in on how to get it working?
Concerning iMovie 10 what do you mean by "the new version is blank"? Have you tried updating projects from iMovie 9? (Doing this will not damage your iMovie 9 projects and event folders or prevent you from using them under iMovie 9)
Geoff.