It was mentioned before that they are ancient - IMHO they are so old, that they don't expand their empire any more, they don't develp new technologies, and their whole economy and society is as static as it can get...
They are so old they probably messed with their own evolution - beyond cybernetics, genemanipulation they also could have created a conscience-net.
Their very brains could be hooked onto a network that way the manner in which you speak of a Shivan and all of them becomes blurry.
There is a conscience of the idividual, however said conscience could partly exist in the whole network, or in others as well.
It could be that thanks to this dissolved state of individual and personality the Shivans don't know death as we do anymore, and the question wheter there is a Hive mind is just as pointless as wheter a Shivan is a single individual, a projection of the whole, or a part of the group mind.
Possibly a Shivan can exist on its, however its own speciman could view such a creature as alien or severly hampered, the way some people speak of invalid and disabled persons.
Possibly separation can be a great trauma for a Shivan, or a Shivan who never experienced the "network" can never develop some mental facilities that are common to the rest of his specie.
For someone like that, us individualistic vermin could be an eyesore or just a selfreplicating virus that may distrupt the network or devolve the Sain Shivan Existence.
Their technology reflects this attitude - I don't care what comes I always preveil, there is no such thing as failure or end of story - so they could build their ships and stuff over centuries, wander for generations in empty space, and then wreak havoc just to ensure their transistency.
Probably if there ever was a Shivan home world or a bound Shivan it could be forgotten or they themselves view the though of bound individual Shivan as sacreligious, and deny the fact that their origins lie inside the gravity well of a planet.