Indexed REC-19: Is there any relationship between the hub selected and the registry selected (e.g., if PJM-NI, must registry be PJM GATS)?

No. According to the RFP Rule Section V.5.4, a Bidder shall choose an “Indexed Hub” of PJM-NIHUB or MISO-IL, and this selection is independent and regardless of whether the project is or will be interconnected with MISO or PJM.