Yep QFT. I also heard Megatron yells out Devy's name off-screen.

 

In the movie, Devastator started with 7 vehicles and after that a few more vehicles seemed to be added.
Overload, Scrapper, Mixmaster and Hightower were neatly lined up. Scavenger was behind them in the right position to pull them. Long Haul and Rampage (the legs) came over from the top of a ridge.
After that, Devastator added a left hand from what looks like another Scrapper.
The parked Long Haul (another Long Haul) started transforming also but they don't show where he went.
So there were at least 9 Constructicons but maybe there were more that were used to form Devastator.