Sorry for the late reply.
I think I did cover the relationship of operating unit and customers/suppliers.
Customer and Suppliers are visible to the whole installation.
Any thing under it is visible only to the specific operating unit.
You can share customer accounts among different operating units.
Keep in mind, the account is share, not copied.
Removing an address from one operating unit will also remove it from the other (as it was shared).
You seem to be on the right track.