Yeah, I remember the mention of "ships the size of planets" and "symptoms of a bigger problem" comments. I brought them up in a thread one time. I imagine you would go inside these huge ships. Maybe "ships the size of planets" is just an exaggeration meaning very large ships like Gargant size.
I guess the real reason Volition put those hints in about what the Shivans are and their origins is not because that is really what the Shivans are supposed to be, but to keep you guessing. That is perhaps why they seem not to make much sense if you combine every hint together and say they are all that. Maybe only some of the hints are true, like Bosch saying they are very old (what if there had been countless races ...... each annihilated by the Shivans) and the Ancients believing they are from subspace, and the tech database saying they were likely constructed by another entity. When you hear the word "entity" it could make you think they were created by some supernatural force, like how they are portrayed in BP, though it probably means another race created them since FS is purely sci-fi.
Actually... if you scroll a few pages back... then General Battuta analogy about the Shivans possibly filling a similar function as "white blood cells" do in human bodies, pretty much wraps up everything that is known about them in a logical fashion, including... especially rather, that final comment about them being the "symptom of a bigger problem" lol
If something along these lines is what
had in mind or what their final solution would have been is anyones guess... . As far as ingame information goes i think it's also pretty clear that you have to distinguish between actual presented fact and speculation of a character or entitiy within the story. But in any case... it actually IS quite possible to fit the pieces together in a coherent fashion, even tho we don't know if the result is anywhere close to what
actually did have in mind.