Maybe you can use the vids from rubboarder.
There are three vids:
Link#2 gives an explanation how to play chords.
Link#3 is J'ai passe in C and there's also a version in G

You are right. On a YT clip I saw/heard Denus McGee play Mon Chere Bébé in D (or C if you like). So I guess that we can conclude that there are no rules at all.