Yes. I too think that the Shivans had different priorities between FS1 & FS2.
And, while I'm figuring that the Shivans are afraid of us (not just us Terrans and Vasudans, but all gravity-based life who learn to use subspace because of the threat we pose to them by entering their realm), but I just now thought of something else for why they (or how they can) send so many to die to stop us... what if they're clones?
And now I'm not going into any silly "attack of the clones" thinking here. But what I'm thinking is that, even if Shivans are individuals, what we're fighting are the clones of individuals. You're a well-trained Shivan fighter pilot, but instead of you going out to face the enemy, you clone yourself (along with your memories and personality), and send those clones off to do battle. If they die, no problem, you just clone yourself some more. If they don't die and return, you take in those new memories and experiences to improve your skills.
But then I just thought up of this idea just now.