Can i see an endodontist for a consult (vs. periodontist) if a fistula is a result of tooth decay at/ inside the margin of a permanent crown?

Yes: That is the appropriate Specialist to see for this issue.

You can: Regardless of the cause of the infection, you need to look at the big picture: is the bony support strong enough to warrant extensive care on the tooth. If not, then why pay for root canal therapy? Is it an endo-perio problem, or a perio-endo problem?

Endo first: If the tooth has been evaluated by your dentist and it is truly restorable you always do endo first before perio. Sounds like the leakage/decay caused the tooth to become abscessed to the root canal should be done first and any other bone/tissues issue would happen after the infection/root canal is handled.

See an Endodontist: Diagnosis can be challenging. Endodontists are trained to differentiate and treat complicate cases.