Thanks, Granite26. Somehow I missed that discussion the first time around. Assuming that the size-0 parts will be effectively non-present, that would indeed compensate for a lot of the functionality/body-part issues that I was thinking of in my original post. Having a way to control the way that creatures grow, as opposed to the magical kitten->cat transformation, would be awesome.
That being said, it seems like a lot of other life-cycle issues have come up in this thread, most of them awesome. It would still be cool to have multiple options for what creatures turned into (as suggested by alfie275), with antman larva -> antman worker/warrior/drone/queen. It would still be cool to have creatures turn into/from vermin, as with the cave spider / giant cave spider switch. And, while the growth system could (in theory) handle multiple stages of creature development, it seems like that would be awfully cumbersome to implement -- eg a caterpillar with 30 limbs turning into a pupa with no limbs, then a butterfly with 6 limbs.
Originally, I was thinking that having separate RAW files for the creatures would be the easiest way to provide flexible, in-depth control of all of this, but now I'm not so sure. At any rate, it certainly doesn't look like the development is heading in that direction. Anyone have any thoughts as to how some of these features could be (easily) added in?