Hi Jesse,
PR and PO both can be transferred to the SAP ECC backend through RFC and hence PI is not necessary. It is only if you need SRM to connect to non-SAP systems (& SAP systems like SUS), you require PI.
Please set-up an RFC connection in SM59, job(as mentioned by Robin) and the other settings like n umber ranges, transaction types and back-end document types you should be good to go.
Hope this helps 
Cheers,
Yateesh