nexus S partition scheme, can it be changed?
Not sure this helps you 100% as I've never done this on the Nexus S but I did it on the HTC Desire, which the manufacturer gave a laughably small area for user installed apps.  I follows this tutorial:

http://alpharev.nl/

Basically you're using fastboot to put a new hboot down. I suggest googling for more info about these; whether there's an existing one close enough to your requirements, or if it's easy to create them yourself.