Yes, each device can be managed independently from others since the device preferences follow the device. So for your Nano you have several options:
1- Manage the Nano manually, i.e. don't auto-sync everything in the library. You literally drag and drop what you want on the Nano from the Library (songs, playlists, video, etc.) I'm not much of a podcast guy, but I think you can auto-sync podcasts but manually manage music and video content. The downside to manual management is that activity on the iPod does not transfer back to iTunes. For example, if you rate a song on the Nano, that rating stays on the Nano. If you play an album on the Nano, the playcounts for that album will not be incremented in iTunes (if that matters to you).
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2- Create a simple playlist that you manually manage, then auto-sync just to that playlist. You can drag and drop music to the playlist as you see fit, then upon each auto-sync any rating changes or playback history from the Nano will be reflected in iTunes.
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3- Create one or more smart playlists that automatically update based on your ratings and playback history, then auto-sync to the smart playlist(s). This is helpful when your library exceeds the capacity of your iPod, but it is also helpful if you want to "steer" what goes onto the iPod based on ratings or playback history. For example, why have a bunch of 1- or 2-star songs taking up space on the iPod? Or, as your library grows, maybe only put music you haven't listened to in at least a week onto the iPod. For more about this method of managing an iPod, take a look at this document written by Code Monkey, a user here at iLounge:
http://www.keepandshare.com/doc/view.php?id=1154607&da=y
Additionally, for podcasts and video content, you can set up auto-sync preferences to only sync selected items, or un-watched items. These settings are independent of how you set up your iPad.