This is my own personal theory about the Shivans.
(NOTE: This should be in no way, considered to be cannon... err... canon)
Origin of the Shivans
Part 1: How do Shivans come into existence?
Shivans do not come into existence like like other beings, they are not "born". Shivans come into existence as fragments of subspace, created when subspace itself is shattered, or to be more specific, a section of subspace.
Part 2: How does subspace get shattered?
Subspace does not naturally shatter, however if a large amount of stress is placed on it, this can happen. Normal subspace travel may cause stress, however it is negligible, however if something goes deeper and tries to force open a subspace portal then the stress is greatly increased. Basically the reason subspace was shattered and the Shivans created was because of the Knossos and the stress it placed on subspace.
Part 3: What happens when subspace is shattered?
When subspace is shattered the fragments start to emit varying frequencies of quantum pulses (which I will now refer to as QPs) and of varying intensities, as the QPs are emitted they come into contact with other fragments, if the intensity of the QP is higher then the intensity of the QP the fragment is emitting then that fragment will start to change based on the QPs frequency and the size of the fragment, the very small ones become Shivans, with anything larger becoming one of the Shivan's ships, this is why there is a large diversity in Shivan ships, however, since the QPs the fragments can travel extremely far at higher intensities, and the fact that when a fragment transforms into something it releases a QP burst, there is some constancy among ships.
The First Terrans Shivan Invasion
Part 1: How did it start?
The first Shivans were created by the Ancients when they created the Knossos, after this happened they were overpowered and destroyed, after which the Shivan's traveled the galaxy until they found the Terrans and the Vasudans. Since Shivans are hostile by nature, they attacked. Basically, when we first fought the Shivans, we were just cleaning up after the Ancients.
Part 2: Why was this fleet smaller then the second one despite the Ancients using subspace for millions of years and being unable to kill the Shivans?
Remember that normal subspace travel only causes minimum stress on subspace. The problem comes from when the Knossos tries to force open a node, so traveling through a Knossos wont cause subspace to shatter, only its activation. Still, the Ancients probably activated multiple Knossoi, however, the reason that the first wave was so small, is because, shortly before the Ancients were eradicated, they closed the Knossos, hoping to keep the Shivans from eradicating other species, but there were still some on our side of the portal, that is wave we first fought.
The Second Shivan Invasion
Part 1: How did it start?
The second war with the Shivans was caused with the NTC Trinity activated the Knossos for a second time, causing a part of subspace to again be shattered and fragments to be created, in addition to that, there were also the Shivans that were trapped behind the Knossos by the Ancients to contend with.
Part 2: What is the Sathanas?
The Sathanas is basically a massive subspace fragment. Normally a fragment of this size wont become a ship because it emits a high intensity QP that cannot usually be overpowered by another one, however, this time it was and the Sathanas came into being.
Part 3: Why did the Shivans capture Bosch?
The Shivans did this because they wanted ETAK. From what we know, ETAK modulates QPs which, according to my theory, controls what forms subspace fragments take. However it goes farther then that, by controlling QPs you can also cause subspace to break down further, causing more fragments to be released. Due to this, the Shivans captured Bosch to gain control of ETAK to expand their forces.
Part 4: What are the Shivan Comm Nodes?
I don't know how they did it, they could have stolen ETAK from the Iceni, or maybe Bosch gave it to them, either willingly or after torture, but the Shivans got control of ETAK. The Comm Nodes work like ETAK, modulating QPs and are designed to do so in a manner that will cause subspace to break down, and more Shivans to come into existence.
Part 5: How did the Shivan's create the Comm Nodes so quickly after acquiring ETAK?
The reason they were able to make Comm Nodes so easily after acquiring ETAK was because of the nature of it. First, the Shivans used the technology on the Iceni to activate the Knossos from long range , then they used ETAK to fire a long range high intensity QP blast with the frequency to create the Comm Nodes. Since the range on ETAK is unknown this could be possible, but due the the fact that it was at range, the effect deteriorated and only 3 fully functioning Comm Nodes could be made.
Part 6: How did the Shivans get so many Sathanai?
When the Shivans acquired the ETAK technology they also acquired the knowledge of how to activate the Knossos, and with a few modifications, they did it in such away that many lager fragments were created. With this technology, the Shivans opened the 3rd portal (the 2nd was opened to make the Comm Nodes) and used the Comm Nodes to create high intensity QPs to allow the larger fragments to become Sathanai.
Part 7: Why blow up Capella?
After acquiring ETAK the Shivans outfitted a variant of it on the Sathanai (doing so was as easy as a slight change in the frequency of the QPs used to create the Sathanai). Then they headed to Capella with this army and used the modified Santhanai to blow up the Capella star in such away that it caused large damage to subspace and created many more Shivans, perhaps more then we could imagine. This action was taken even with the Comm Nodes under thier control because the Comm Nodes work gradually, whereas this could created billions or more Shivan's in only 72 hours.
Part 8: Why did some of the Sathanai enter subspace after triggering the supernova?
I can think of 2 different reasons for this. Either they thought they could "dodge" the explosion by entering subspace, or they were pulled into supspace against their will by some kind of force.
Aftermath
What happens next I do not know. Maybe the Shivans use the Comm Nodes to force open a node to GTVA space. Maybe they head to a new Galaxy through the 3rd Portal and wipe out countless more species, or go around blowing up every star they can find until there are billions of Shivans. Maybe the GTVA builds a new Knossos and opens the node to Earth, only to watch as the Shivans created by this portal eradicate the GTVA from existence, or perhaps when the Lucifer exploded, it cause that part of subspace to shatter, and the Shivans brought into being from this have already brought the Earth to an end, or the battle could still be on going.
The End
I hope you like my theory, maybe it helped you come up with your own. Take what you like from this, I know I make a lot of assumptions, but this is what I have come up with. Please give your opinions on this. And if you have any questions I will do my best to answer them.
