You may want to put together a convicing rational for why the Blimps have chosen the lighter-than-air path. Here on Earth we have no evidence for any lighter-than-air animals, and yet we have at least four evolutionary paths that have discovered flight by wings: insects, pterosaurs, birds and bats. This suggests that we are unlikely to see Blimps on a terrestrial world in a hard SF universe.

The only rational that comes to mind would be pressure towards a very slow metabolism. Winged flying tends to be pretty high-power (though geese still fuel continental migrations on grass alone). If there's little to eat, then maybe floating would make sense. A slow metabolism would of course radically change the gameplay aspects of this race.