ok, heres my theory from the top:
Shivans evolved orignally on a low-g planet/moon, hence the generally small body and long limbs. This also would make it easy for them to adapt to 0-g. When they became a space fairing race, they eventually adapted to zero-g. Their movement in 0-g conditions and their long limbs support the theory that they are specifically adapted to such an environment.
This does not rule out the possibility of artificial enhancement, and certain characteristics of shivans go a long ways to prove that. The nice spiffy plasma cannons they mount on their shoulders is most definitely artificial in origin, yet the way they are attached to the arms suggests not a mear snap-on attachment, but a more permanent one. Also, the armor plating on their bodies do not appear to be organic,
, they are very metalic looking. This also suggests that the shivans are possibly enhanced in ways more complex than cosmetically applied plating.
It is very possible that shivans are cybernetically enhanced to be speciallized killers. Their control over the shoulder mounted plasma cannons suggests at the very least a cybernetic interface allowing the brain to send commands to the weapons without the use of an appendage to pull the trigger.
As for the hive mind idea, if shivans were composed of a hive mind then there would be no need for communications subsystems on their ships, or comm nodes because they would already be connected. Also, shivans were highly disorganized after the destruction of the Lucifer the tech archives say, were they a hive mind then they would still have been able to reorganize and cooridnate.
Thats my two cents, nothing more.
158th Banshee Squadron http://3dactionplanet.com/hlp/hosted/the158th/
Into the Night Campaign http://3dactionplanet.com/hlp/hosted/the158th/itn/