As Sky Shadow said, it boils down to the personalities of the individual gestalt members and also how their personalities interact with each other. One interesting feature that the Micromaster tech specs had was ratings for "Teamwork" and "Cooperation." Gestalt teams who have higher degrees of teamwork and cooperation combine to form more psychologically and emotionally stable gestalt robots (e.g. Protectobots, Predacons), whereas those with poor teamwork and cooperation combine to form mentally unstable psychopaths (e.g. Constructicons, Stunticons).

I wouldn't say that Predaking is necessarily stupid -- he's no Rhodes scholar, but I wouldn't say he's an imbecile either. The tech specs basically describes him as the perfect fighter who moves with feral animal-like ferocity and precision. One would imagine that watching Predaking in action would be like watching a massive predator like say a Great White... moving with surprising speed and savagery that belies his immense size. Each individual Predacon may be incredible fighters in their own right, but when combined they become an incredible fighting force to be reckoned with... it's like making a giant boxer by combining Bruce Lee, Mohammed Ali, Chuck Norris, Alexander the Great, and King Leonidas... phwoar!