The question has been put forward though on TFW; "Should RiD/Car Robots Megatron be 'shoe-horned' in with BW/BM Megatron?".
I wouldn't include him. Lumping RiD Megatron with BW Megatron isn't any better than lumping him with G1 Megatron - in the end they'd still be lumping together two entirely different characters who happen to be called Megatron.