Actually, I did not find enough information in your question to give a complete answer. Instead let me explain some of my personal lessons learned and see if any of this helps.
ITUNES is an application that runs on a primary host. The hose can be a Win box as in my case or any Apple OS computer.
The ITUNES host imports data in the form of music, movies, videos, pictures, etc. The catagories listed in ITUNES which you can find on the ITUNES website.
When ITUNES imports data, it adds the fact of the data to its library. The lilbrary is a listing of the information about the data, song title, album, cover are etc. The library exists on the ITUNES host computer in the ITUNES folder.
When ITUNES imports data, it actually saves off the data to any location that the user establishes in the ITUNES preferences. The default is on the host hard drive in the ITUNES music folder, but you can point ITUNES at any hard drive on the network accessible by the ITUNES host. For example, I used to host the library and the data on my ITUNES host, a purpose built XP box, but the library rapidly out grew my host disk size and is now resident on an external server. A consideration is the size of your library and the cost in labor hours and dollars of replacing the ITUNES data. If you loose the data then you are out of luck. ITUNES will ONLY allow you to download the data ONCE and if you loose it you are out of luck. You can back it up to CD or DVD if you have a burner but once you reach a certain size it can take for ever to back up. Right now my library is 3+ TB and it takes most of a month to manually burn DVDs to back it up. Thus I have two servers for my library data.
Once you have ITUNES hosted on a PC, that PC can share music, videos etc with up to 5 authorized machines. this means the host plus 4 other computers. There is no limit to the number of IPODs that can be associated with an ITUNES host, but you can ONLY share with 4 other PCs. Believe me I learned this lesson the hard way. I maintain all my movies and music in duplicate files so I can share them with unlimited computers via WMP but only 4 other PCs are associated with my ITUNES host.
When you plug an IPOD into an ITUNES host for the first time ITUNES "remembers" that IPOD uniquely. You need to name that IPOD with a unique name - Rob's Red IPOD - for example. Then every time you plug that IPOD into that ITUNES host, Rob's Red IPOD will pop up and you can sync it.
The first time you plug in that IPOD, you set up syncing for automatic or manual. If automatic, then ITUNES will randomly select from your available music a set of tunes and load them onto your IPOD till its filled up. If you set it for manual, you can then drag individual tunes into your IPOD (or video) or you can drag whole playlists into the IPOD and they will be synced.
If the combination of playlists and video's is larger than the IPOD capacity then ITUNES will give you an error message and you can either shrink the size of the playlist or select less video content to load on the IPOD. Once the list of items to be added is at or less than the capacity of the IPOD ITUNES will execute the sync and you are good to go.
If the IPAD is treated like an IPOD, then you can sync it in a manner similar to syncing an IPOD.
If the IPAD is treated as a computer rather than as an IPOD, then you need to have an instance of ITUNES running on the IPAD and you can share with the ITUNES primary host. This means you can play any music, videos etc on the ITUNES host if you car connected or you can drag copies into the IPAD resident ITUNES instance and they will play as long at that IPAD ITUNES instance is one of the 4 "authorized" on your ITUNES account.
Or you can treat the IPAD as its own host for ITUNES and set up a separate account with ITUNES. But then you can not share purchased or DRM protected files between the two ITUNES accounts.
Alternatively, you can do what I do and that is save off your music and non ITUNES purchased items into a separate file, I use WMP, winamp or other tools to import CDs and DVDs and save them off in MPEG4 or .WAV files. You can separately import these data files into the second instances of ITUNES and they will run just fine.
I would NOT recommend making any portable device your primary host for ITUNES purchased data. It will get broken or lost or . .. murphy's law says you will loose it when it hurts the most.
Hope this helps