Well I believe it's clear that they did not develop on a planet.
This is a false conception. The basic truth is we do not, cannot, know, since the Shivans are all now cybernetically enhanced. Yes, their form as we know it is singularly well suited to zero gravity, but that proves nothing about their original form.
That's just it: we don't know. To virtually every question about them, we just don't know. For that matter, we don't even know for certain that the monsters that attacked the boarding party in the FS1 cutscene were Shivans (and not part of some automated defense system).
For that matter, do we know that there
are Shivans? We know there are Shivan
ships, but can anyone recall clear evidence, in either of the retail games, that those ships are actually occupied?
While I don't think there's any definitive answer provided by the games, I'm fond of a few theories:
-The Shivans died a long, long time ago. The war machine we encounter is just that: a complex autonomous armada built to wage war against an enemy long dead. Unfortunately (especially for Vasuda Prime), the identity of the "enemy race" has detiorated with time to become less and less specific.
OR, even more fun:
-The Shivans are aware of a unique threat to the cosmos, one of simply terrifying scale. This threat resides in subspace. Early subspace technology hasn't granted the GT(v)A enough freedom to discover this threat, but future advances in the technology would. As races gain more and more access to subspace, their is a greater and greater chance that they will disturb this slumbering giant. The Shivans will not allow this to happen, and preventing any race from accidentally unleashing this force on the universe is their sole goal, to be met by any means necessary. The Shivans are spread over a great distance, meeting potential threats (i.e. subspace capable races) with whatever force they have available at that location. As threats become more evident, they shift their forces... but it's a lot of ground to cover, hence the 30 year gap between the great war and the second shivan encounter, which had much more military force. The first war made it clear to the shivans that they needed to allocate more resources to the fight. Annihilation is not the true priority, however, and sometimes culling species with subspace tech needs to be put aside when a higher priority objective is present. The supernova at the end of FS2 was part of a containment operation. The Shivans aren't the Destroyers. They're the Gatekeepers, closing doors that should never be opened.
Here's a funny little bit:
Remember in FS1 when a Vasudan pilot comments "They're unstoppable. The old stories are true!"
What if the stories are true, but they aren't about the Shivans? What if the Shivans are trying to prevent us from jumping through subspace one time too many and catching the attention of..... ?
Anyway, that's what I like to think while I'm trying to line up a shot on a Mara ;-)