cck Nodereference for files
You don't want FileField Sources, you want to use a Node Reference field on the node you want to reference the file on, and you want to change the display of the node reference field with either a Custom Formatter (that you'd need to write yourself) or a standard FileField via the Formatters 4 Node Reference field module.

To change the display of the Node Reference you would want to goto the Display Fields page on your Content Type and select the new Formatter from the dropdown.

A brief demo of what you could potentially do for this sort of system can be seen at: http://www.youtube.com/watch?v=qBXoN25guic