Cmd-Up and CMd-Down are used for navigating the folder hierarchy in the Finder, but you can use them in FCP X without causing any conflict. There are two ways to do it:
1) By using FCP X's command editor --- this is the preferred solution
2) By using System Preferences->Keyboard->Keyboard Shortcuts and adding the desired shortcuts. The name of the commands are exactly "Beginning" and "End", so that is the exact text you'd have to enter. Shortcuts entered this way need to exactly match the name of the menu they are to be associated with.
I mention option 2) here because while it has been there since OS X exists, and yet it seems to have gone conspicuously unnoticed, and it IS useful even for Final Cut Pro X.
Why? Because there are menu commands for which the Command Editor does not offer the possibility of adding a shortcut.
This is the solution that even Steve Martin failed to notice when asked recently in the virtual user group about adding shortcuts for exporting to different destinations. FCP X only offers one default destination (for which Command-E is the shortcut), but it is in fact trivial to have shortcuts for several different destinations with this technique.