No, it's not true. Mine would be empty if that were the case.
As long as the files are a format that is supported by the ipod (MP3, AAC, WAV, and a few less common ones), it will play them regardless of wether you've ripped them from your own CD collection, downloaded them from illegal peer to peer sites, or purchased from online stores.
It is possible to use 2 ipods on the same computer. ilounge is really good site for finding out about this stuff.
An ipod ransfer is needed. You can transfer songs between ipod and mac with this software tool. Media URL: http://www.ipod-to-pc.net/transfer-music-from-ipod-to-pc.html Description: