I wonder how this will affect Devestator. Will he not exist?

Well, most gestalts have five members, not six, so presumably he could exist, but he'd probably be smaller, and different in personality. (As much as a gestalt like Devastator has personality, anyway.)

And he managed that because the Decepticons thought he was a harmless, naive, absentminded youngling.

Oh gee, who could that be? It certainly doesn't sound like a certain Microscope...< /Sarcasm>
Of course, Percy probably was a harmless, naive, absentminded youngling. He just happened to be a harmless, naive, absentminded youngling with classified documents and a planet-god besotted with him.

"Oh, look. Xantium and Metroplex are arguing again."
"What's it about this time?"
"I have no idea."

Xantium-Kitten insists on poking Metroplex-Cat, who's just settled down for a nice nap in the sunlight...