...And no one has addressed downloaded music. E.g., iTunes allows you to download your purchased music once. If it's deleted from your iPod, you'd need to buy it again.
bobtomay is right: ". . . for AK47 . . . Try creating a Playlist(s) for what you want on the ipod and then use the Music tab to sync only that playlist."
Syncing is great. It only syncs one way - from your computer to your iPod. Your iPod cannot delete anything from your computer (or external HD, as the case may be). From what bobtomay said, you can have a huge iTunes library, but a playlist called "iPod" (for example) that you move music in and out of as you wish, syncing only that playlist to your iPod.