Demanding logic from Kaiju Eiga? Where is the logic in that?
I'm all for movies, that make you think (2001: A Space Odyssey and Blade Runner are among my favourites), but expecting this from Kaiju Eiga is misplaced. They are primarily about the visual spectacle (the fights and the destruction). Only a few go beyond this and raise some issues (like my favourite movie, the original Gojira).
In any case, what has this topic with GalCiv 3 to do again?
Because in GalCiv III, you will be able to recruit giant monsters to fight alongside your marines. Godzilla provides some great fire support!
I'm being silly of course, but who knows what those Drengin might come up with. They enslave most other races, so why not the occasional giant Turtle that is capable of flying at Mach 1...