: I am wondering if there are compatibility issues with using:

Epoxy resin atop poly resin? NO!

Poly resin atop epoxy resin? Sometimes, but I wouldn't do it.

: Is one a no-no and one OK? Nope, neither is a good idea.

The failure is often slow to show. Epoxy over polyester seems to be fine initially, but often fails within a year. If you keep it dry, it helps a lot (not too likely on a kayak). Poly over epoxy sometimes works but why would you? Polyester Gelkote over epoxy will almost never harden, though the gelkote guys at the boatyard where I work keep trying.