[On the topic of the Gargant]
Consider it like this; it's currently common belief that the shivans operate as a collective (like an ant colony or bee hive).
Now in FS1 you had the Lucifer commanding the fleet, and when it was destroyed, the entire fleet broke down.
I reckon the Gargant is
the flagship of the Entire Shivan fleet (with the Gigas's as fleet flagships). Thus, if the Gargant was to somehow get destroyed, their entire race would devolve into anarchy.
It's important to note that the Shivans probably know as little of us as we do of them, so for all they know, we could have a way of destroying their flagship (like we did in FS1), even though we don't. So, the Gargant is left behined and used to defend Shivan space while the rest of the fleet mobilises.
This has the added advantage that the Shivan fleet always has falling back as an option, becuase the Gargant can always be mobilised to smash a blockade or enemy fleet that is hampering the retreat effort and then pull back into Shivan space before the enemy can mobilise an effective defense.
So in basic terms, the Gargant is a command centre, and makes sure the Shivan fleet can pull back if needbe, but is too important to be risked in an offensive operation so far into enemy space.
Besides, its too F****** big to be of any tactical use. I would rather have the Gigas...with a 6-Sathanas escort
Sorry about the long post, shoot me later.