So if I understand you correctly, you want to be able to sync only certain songs, artists, and albums correct? If so you can configure this set up from under the iPod's Music configuration tab in iTunes as described in the Syncing Automatically section in the Apple support document you linked in at the top of the page. Note that you cannot select only certain songs, but you can set it up to sync selected Artists, Genres, and Albums.
If the tab and its configurations are grayed out, it is likely because your iPod is set up for manual management, meaning the manually manage music and videos option is enabled from under the iPod's Summary tab.
B-rock