Well... Bosch is also a great one to base... Their are in service as a pilot in the Intrepid in FS1 (but none know the exsact moment where their joing the Military...), then their go to Intelligence pilot afer the Destruction of the Lucifer, then comes the dark part... the 32 years period betrenm the destruction of the Hades and FS2... when their reach high rank and created a rebelion, and stolen/get some informacion about a GTI Project involving a captured Shivan Specimen... then their start the rebelion and cause (none knows if intentional or not) the second retrun of the Shivans...
The only problem... none knows from where the Shivan come in FS1... im not sure if this is needed to make the series... but... all about this... all players have a diferent idea about this... the original point of the aparence of the Shivan... the only knowed about this is that should be in Ross 128... but what caused to come... none knows..
-I have a theory about this, is what I using in a campaing that I making-
The first come of the Shivans has been a GTI fault, not intentitonal of course, their go to Ross 128 for make a field tests of a few new prototypes of ships and weapons, they selected Ross128 for make a low profile, their dont whant that this tests could oberseded by Vasudans

, a(their go to Ross 128 which other GTI science ship, a faustus, the Hinteing, (who is mencioned later in Silent Threat), the Ships are carried in a experimental new corbette ship... Well, the idea is, that the GTI go to a Nebula which asteroids to test a new class of Anti-warship bomb.... but what whappend is the their addidentaly hit a part of a hidden knossos portal (their dont see it), and activated it, when the portal start to work, their re activate the subpace corridor to a other system, but the asteroids in the field crash and destroy the portal very fast, so the subspace as very unestable, but the Shivans can trip in a unestable Subspace... GTI never, known what their do... the only that their see is a piece of the portal and the BOOM! their never known what causes... or what their are... and because this information as very classified, none learn about this accident... later, afer their exist Ross128, in the midle of the first Shivan attack on the installation,the corbette that carries the Ships is ambushed and destroyed in Delta Serpentis and the Avenger cannon is stolen from it...
- But this is only my idea, and very sure that everyone here have his own theory -