can i install just Ogg/vorbis and nothing else?

[toupee]

Can I install just a few parts of Rockbox on my ipod - for example, just the Ogg/vorbis codec - leaving the rest of the players manufacturer firmware intact? Or do I have to install and use the whole Rockbox system to get any one thing?

[Chronon]

The native firmware will have no way to use the codec.  In order to gain access to Rockbox's functionality you need to install it according to the instructions.  It's a self-contained firmware written from the ground up, not a set of plugins designed for a pre-existing firmware.  

You could probably get away with a sparse installation of Rockbox, but if you're going to take the plunge I don't see why you'd want to cripple a superior firmware (Rockbox!).  Remember that you can always boot into the Apple firmware if you so choose.

[stripwax]

The vorbis codec is just "a part" of Rockbox.  There's no way to install any new codecs into Apple's iPod firmware, sorry.  (There is no documentation available on any kind of codec interface for Apple's original iPod firmware)