In the first Transformer film, many Decepticons have military/police alt-modes, whereas all Autobots have civilian alt-modes. Is there any rationale behind this ? Or do the alt-mode simply reflect their personalities ?
First, we should address this claim:
Let's go down the list of Decepticon alt-modes from the 2007 movie:
Even just ignoring the mutations, we're only at 4 Military-Based Decepticon Alt-Modes, or half of the 'Cons.
But, that's just nit-picking. Why do the 'Cons have military-based alt-modes while the Autobots have almost all civilian-based alt-modes?
The Decepticons have been on Earth for an undisclosed amount of time, but it is clear that they arrived before most of the Autobots (Bumblebee being the only possible exception). This means they've had more time to pick and choose alt-modes more suitable to their mission, and more to their liking.
The Autobots, on the other hand, (again, excluding Bumblebee) arrive pretty much at the last minute. They are largely forced to adopt alt-modes based on the first vehicles they encounter.
Since the movie universe does not include any mass-shifting, the Transformers must adopt alt-modes which have (roughly) the same mass as their bot-modes. This is the only "rule" which they must follow when choosing an alt-mode.
This is pretty much just a nod back to the original G1 stories. In the original cartoon and comics, Teletran-1 chose the alt-modes for each Transformer. For some unknown reason, Teletran-1 sent Sky Spy to a nearby military base to choose new forms for the Decepticons, but to civilian areas when it came time to scan modes for the Autobots. Exactly why Teletran-1/Sky Spy did this is never explained, but it fits the theme that the Decepticons are a war-mongering violent faction, while the Autobots are a peaceful, non-militaristic group.