Dear,
yes ,We can have different document types for different plants.
Kindly check with User Exits COZF0001 and COZF0002, with this you can have different P.R document types even though if you have configured default P.R document type for external procurement.
Kindly read teh SAP help document on the same
Change of Purchase Requisition - Customer Service (CS) - SAP Library
Reagrds,
Pardhu