I guess this follows the comic books? Isn't Galvatron in charge of the Decepticons after Megatron was slain...that's the way the movie from the 80's went at least....or are you going from the Micheal Bay transformers?
Regardless, why not have Galvatron secretly be on earth running the show, or just use one of the combining robots like Bruticus and let him be in charge because he's military...perhaps he figured out how to colonize humans after Megatron was defeated... man I hope I'm doing this right. LOL