Does an incumbent exclusive representative need to submit any particular documents to intervene once a petition has been filed?

No.  An incumbent exclusive representative is the labor organization that is the current exclusive representative of all of the employees in the unit sought.  An incumbent exclusive representative will be considered a party in any representation proceeding raising issues that affect employees the incumbent represents, unless it serves the Regional Director with a written disclaimer of any representation interest in the claimed unit.  See Section 2422.8(d) of the Regulations.  Although not required, an incumbent exclusive representative should submit a copy of the contract or certification.