I'm not sure why any of this would affect your five "uses" but if you want to exclude certain songs from being copied to your iPod, there are several ways to do it. I'll outline one way, and maybe it will give you some ideas.
You can set up a smart playlist that has the conditions "Comment | does not contain | <some keyword>". The keyword can be anything, such as "exclude." Then you just select the songs you don't want on the iPod and add "exclude" to their comments (remember that you can select multiple songs and edit their comments all at once). Finally, instead of having your iPod automatically sync with your whole library, you go into the iPod options and have it sync with just the smart playlist you just created. The only problem with this technique is if you have other smart playlists that you also want on the iPod, you might need to add the "Comment | does not contain | exclude" condition to them so they won't include the excluded songs.
Of course, it's possible to manually set up a playlist of songs that you *do* want on your iPod, but that would require you to manually add songs to the playlist every time you add new songs to your library. The advantage of the method above is that new songs added to your library will automatically be added to the "non-exclude" smart playlist. The only time you'll have to think about this at all is when you want to prevent more songs from being on the iPod. Then you will just add "exclude" to their comments. Does that do what you wanted?