Should I delete large backup FCPX files?

If you are talking about the backups that FCP stores by default inside ~/Movies/Final Cut Backups/ then as LocaAlicia stated it's usually safe to keep just the newest backups, unless you think there might be a chance you need to revert to an older version for some reason. They are backups only of each library's database and don't contain any media.

Keep only the most recent backup library and delete the rest.

In most Libraries the largest amount of space is taken up by Render files which can be deleted safely as they can be regenerated at any time should you need to revisit the project.