You COULD put 20mm cannons on an F6F-5P, but there's no photo evidence that this ever happened.
Krusty, F6F-5Ps were photo birds, the number of which is unknown as they were not modified by Grumman. Something less than 200, and all were modified F6F-5s, never having had cannons installed (although every F6F-5 had that capability).
I think you screwed up on the designation (you meant F6F-5N) and now you're using that for CYA.
About 1/3 of the 1,400+ F6F-5Ns built were delivered with 20mm cannons installed at the factory. Of those, more than half had the cannons replaced with more reliable M2 Browning machine guns in the field (actually onboard carriers). F6F-5Ns were not reserved for night time use. Almost all flew day time sorties as well. During the Okinawa operation, in an effort to beat off the kamikaze raids, everything that could fly went up, including the night fighters.
My regards,
Widewing