Oh, the Scarabs aren't merely organic in shape, they're essentially just carapaces for a collective sapience created by a colony of wormlike organisms. (This has important gameplay consequences, actually; the Scarabs are invincible except for their Gradius-like shoot the core/fuck-me button which constitutes the curiously exposed colony. The other day I was playing Halo 3 and it took me ten minutes to remember where they actually were--in the ceiling of the inside of the Scarab :angry: ).
The interesting thing is that the Hunters are actually the same life form as Scarabs--they're colonies of worms in a giant humaniform shell, instead of a Scarab "tank." You have to shoot them in the back, where their actual thinking bits are exposed--and this has slightly more justification, since they need oxygen--otherwise they too are invincible. (Maybe the rest of the Covenant should think about building armor out of whatever it is the Hunter-worms secrete.)
I sort of like to think of the Breen in that fashion--not as worms, exactly, but a more a collectivized, crypto-fungal entity whose humaniform refrigerator suits are principally for contact and maintenance purposes. And not different, in principle, from their ships.
Entirely fan-speculative, of course, but it draws on the idea of a frozen planet, which suggests ammonial rather than water biochemistry, which cannot coexist with an oxygen atmosphere, which in turn rules out aerobic life as well as a technological development as we know it. Chris Bennett's squales in Over a Torrent Sea, also bioengineers by necessity due to an aquatic environment (as opposed to a terrestrial but reducing one), helped me think about ideas in this regard.