Are there any rules about compatibility between firmwares and handsets? E.g. is it possible to flash a Black 64GB Swiss firmware onto a White Chinese N9, or a Magenta 16GB Malaysian firmware onto a Cyan 16GB Finnish phone?

Also, would it be wise to ignore (maybe even disable) OTA updates and just do updates manually to the newest (compatible) version via navifirm?

Reading some of these posts it seems Harmattan itself already has quite some fragmentation going on, actively aided by the maker himself (different firmwares (versions) for different localisations, colors or even distribution channels within the same country).