Dear Tamil,
1.SAP NetWeaver PI used as middleware between SAP ECC and SAP Ariba. Download and refer 'Ariba Network Adapter for SAP NetWeaver® Setup Guide' from connect.ariba.com document for detailed description.
2. Yes, Most of the ECC/Ariba documents(PR, PO, PO confirmation, Ship Notice, Invoice, Credit memo) transferred from one system to another easily.
Regards,
Dinesh