Project and Expenditure orgs in multiple ledger/OU environment

Regarding Project Org and Expenditure Org:

We are implementing Project Costing and Billing for 7 countries (7ledgers and each ledger has one OU)

Project Org

Departments which owns the projects are centralised, not specific to any OU. Means one department will cater to 7 OUs. Can we use single organization as Project owning organization for multiple OUs and what is the impact in Project hierarchy?

Expenditure Org.

Similarly we have expenditure orgs which incurr cost for all OUs. Means our Exp. Org are also not OU specific. Can we define exp. org which can cater to 7 OUs? But when we define Project Expenditure/Event Organization, we have to assign related organization (which is particular OU).