The Shivans we're familar with could have been like that from the beginnings of their species. Likely they were engineered to be at home in a Zero G enviroment.
As far the 'no breathing' thing, we breathe to take advantage of the relatively large amount of energy released in an oxidation reaction. We would be relatively immobile if we relied on direct radiation, as our bodies are not set up for it.
The Shivans on the other hand look like they were designed to live and work and war in space.
The idea that they somehow feed on subspace energies is born out by their total focus on controlling the subspace nodes where the fabric of space is the weakest. Their defending their food source.
Also likely, the race that geneered the Shivans were the first to be wiped out by their creations. An act of divine retribution, certainly. But also punishment for creating so fragile a species. Any act that damages or destroys Subspace could, and probably would, be looked on as an act of war.
Using this reasoning, we can assume that the standard Subspace Drives and/or Subspace travel damages Subspace in some way. The Shivans unique connection to subspace means that their drives are different from their Terran or Vasudan counterparts somehow, rendering them safe to use for travel. It would also explair why the Shivans wipe out species that have discovered Subspace Travel. They are being poisoned. What they have failed to understand is that they aren't giving the various species time to discover the flaw and fix it.
If the Shivans are connected to subspace so closely they might not even be capable of understand Humans or Vasudans or that there is another way to solve their dilemma, by providing data proving the existence of a flaw in our design and allowing us to correct it.