About Kryptman's plan to deny the hive fleets biomatter, wouldn't it make a lot more sense to use such a plan as a last resort kind of thing? Make the horde of bugs fight until either the Imperials drive them off or have to pull out, then as the ships move in to start feeding, and indeed, while they feed, we have an Imperial fleet move in burning both the hive ships and the planet to a cinder.

On a side note, this would be the best course of action for the mentioned 'cannibalistic' warfare between fleets. Instead of assimilating the old fleet and getting stronger, the winner gets the torch while eating. It's still scorched earth, but a more controlled scorched earth than just getting the heck out of the way of the fleet and hoping you slow them. In fact, Kyptman's little stunt with the Orks could be used to this degree as well, 'Nids win, tburn the place, it's gonna be a rock anyway, Orks win, see if you can direct them at more 'Nids. Repeat.

Of course, this plan would never make it into the official fluff, it makes far too much sense.