I think you should partition the drive BEFORE using it. When data is written to the HDD, resizing the existing partition will be difficult as the data is fragmented over the whole hard drive. So alway partition on a fresh hard drive.
BUT why not just use the internal hard drive for Time Machine, and then connect your external hard drive with your iTunes library, data etc. to the USB port of the Time Capsule? Then it will show up as a network drive too.
You can even connect a USB hub to the Time Capsule, so you can have multiple hard drives, printers etc. available wirelessly.
I don't have a Time Capsule myself, but many of my friends have, and are doing this without any problems.
So if you're interested, why not take the chance, and if it doesn't work for you, just return it within your 14 day return period?
EDIT:
The other option is to keep your current WiFi router and buy a NAS. It makes upgrades easier, and you can have several hard drives (I have one with space for 4 hard drives). Runs really well hosting iTunes library and Time Machine backups too