Seriously, this isn't that hard. You have to think of Devastator as a single Transformer. Don't complain, no "what about the cartoon" questions, this is different. This is a completely different take on Transformers than the cartoons, comics, etc. Think about it, if it's ok for Prime to transform into a gorilla in one take of the Transformers world, why can't Devastator be a single robot in this Transformer world?
In this movie, Longhaul, Mixmaster, and Rampage are three completely individual transformers. They are not a part of Devastator. Every Transformer can turn into something right? Well Devastator just happens to transform into several construction vehicles. Why is it so hard to believe that he would have scanned the same alt modes as other cons? Or who's to say they weren't dowloaded into him by Soundwave? It takes just a little imagination, and a whole lot of common sense to get this.
Devastator is NOT Longhaul, Mixmaster, Rampage (etc) combined. He's one giant robot with an attitude and apparent craving for sand. That's my take on it anyway.