Thanks Tom. I'll wait until next week to update the OS to Sierra and FCP X to 10.3.3 – after I finish my project. I think 10.3.3 is the latest version, so it's the only version I can get from Apple, correct? You said 10.3.2 is the best, but I think I can only get that with the free trial (which I downloaded a few days ago).
There are directions on the Apple website explaining how to back up the Final Cut app and your libraries BEFORE upgrading, just in case, because the new version will update your library and you can't revert it back. So, if you back up both of those, you can try the new version to see if it works with your current OS.