shared database

Here is the structure of the site I'm setting up. One primary (or default) site with complicated functionality - ubercart, and a host of other feature-providing modules. Four or five child/additional sites with much simpler functionality and independent content. So far this would clearly call for a multisite setup.

I'm not sure if this is simple question or not, but I'm a little bit confused.

I am planning on developing 4 drupal sites that are essentially pretty much the same in terms of what they do, they will look different but will more or less do exactly the same job as each other with the odd exception. What I'd like to know is if it is possible to feed the sites from the same database and most importantly can the sites share the same cck fields 'if and when' they are required, or do you need to either share all fields or non at all?