I would move all my videos, music, and photos to an external drive; you need a minimum of 10 - 15 GB of empty space at all times for the OS to function properly; additionally, if you are editing and rendering video a lot, the general rule of thumb is that you should have at least as much free space as you have files in your current project (e.g. have 100 GB of files = at least 100 GB of free space while you're working on that project). Here are some links to articles on how to move the libraries (which you can then delete from your internal drive after you've made sure they are on the external and working):
I don't have FCPX, but do have FC Express; in the System Settings (File menu), you can designate a disk for all the files, so you could establish a partition or folder on an external and have your files there:
Hard drive getting full or near full?
Do a search for and downlaod and install OmniDisk Sweeper and OnyX.
Here are some of my tips for deleting or archiving data off of your internal hard
Have you emptied your iMac's Trash icon in the Dock?
If you use iPhoto, iPhoto has its own trash that needs to be emptied, also.
If you use Apple Mail app, Apple Mail also has its own trash area that needs to be emptied, too!
Other things you can do to gain space.
Delete any old or no longer needed emails and/or archive older emails you want to save to disc, Flash drive/s or to ext. hard drive.
Look through your Documents folder and delete any type of old useless type files like "Read Me" type files.
Again, archive to disc, Flash drive or ext. hard drive and/or delete any old documents you no longer use or immediately need.
Uninstall apps that you no longer use. If the app has a dedicated uninstaller, use it to completely uninstall the app. If the app has no uninstaller, then just drag it to the OS X Trash icon and empty the Trash.
Also, if you save old downloaded .dmg application installer files, you can either archive and delete these or just delete the ones you think you'll never install, again.
Download an app called OnyX for your version of OS X.
When you install and launch it, let it do its thing initially, then go to the cleaning and maintenance tabs and run all of the processes in the tabs. Let OnyX clean out all web browser cache files, web browser histories, system cache files, delete old error log files.
Typically, iTunes and iPhoto libraries are the biggest users of HD space.
If you have any other large folders of personal data or projects, these should be thinned out, moved, also, to the external hard drive and then either archived to disc, Flash drive or ext. hard drive and/or deleted off your internal hard drive.