You might want to look into using NIS+ or one of the other methods of centralized account management.
As far as sharing partitions, you will want to read through the Linux Filesystem Hierarchy Standard publication on the net. It goes though what can be shared, what can't. Which partitions should be static, etcetera.
It also gives a rational for each recommendation.
You can find the LFHS here:
http://www.pathname.com/fhs/
