Cybernetic exoskeleton / carapace being the external technology which is attached to the biological parts of the Shivans. In my rough analogy, the carapace can be considered akin to the human body - skin, bones, blood, etc - and the biological parts of the Shivans can be considered akin to the brain, organs, etc. i.e. a Shivan is composed of both, in the same way we are composed of both organs, skin, bone, muscle and soforth. So the true for of the Shivans is the combination of both.
Without knowing how (if?) Shivans reproduce, it's impossible to determine which comes first; is the technology first with biologically engineered components grown within, are shivans birthed somehow and then implanted, or are both 'grown' together in, for example, vats or some assembly line like process.
It's probably unlikely the Shivans actively experiment upon themselves; they don't seem to have a need for that, given that they rely upon space combat using ships (ships which are engineered to integreate with the Shivan pilots).
The Shivan strength... seems significant' Hallfight is possibly a bad example, as it's in Shivan territory - they know the layout, the gravity, the atmosphere, and the marines may be hamepered by either the low-G or their gravity boots. But there is an ani of a Shivan ripping apart a Vasudan fighter in space, so they are bloody strong (but not invincible; the crew of the Iceni took out a few of them after all).
