No, you cannot directly partition the Time Capsule. This is not really its intended function.
You can store data on the TC just like any other networked volume, but eventually, given enough time, Time Machine will use all of the space available. Now, with that said, depending on what you do with your Mac that could be a very long time. On my Mac Pro, I have an internal 500GB HD dedicated to Time Machine. Its been going since Oct. 27, 2007 and there is still over 155GB of free space on the Time Machine drive.
If you already have a wireless router and just want a backup device and external HD, why did you bother to buy a Time Capsule? You could have accomplished everything with a USB or FireWire drive hanging off your iMac. You could then partition to your heart's content and share over the network the volumes you want.
As for setting Time Machine backup intervals - there is no direct way to do this (though I think there is a terminal hack); its designed for "zero-click" configuration. Turn it on and it just goes. Backs up just files that change once per hour for a week (or is it a day?), weekly backups for a month, and monthly backups for as long as space is available. Time Capsule is designed as a convenient way to handle backups for a house full of Macs. Yes, you can share other files as well, but Time Machine will eventually use all the space on the internal HD (as its designed to do). If you want to keep a dedicated media partition to share, you can connect an external USB drive to the TC.
However, for advanced file sharing/distribution, the Time Capsule is not the product you want.