I think it's important to be clear on what we know about with that whole conundrum. Creations which gain souls are living beings, but that can range from anything from an animal, plant life or monster to rare familiars like Meteion (although that isn't explicitly addressed in the story because she was never subjected to a proper examination, possibly because the creator's soul aether has to be distinguished from a genuine soul through someone with precise soulsight.) The story never specifies what this "new life" is except for the fact that elements of it had souls, and Venat's faction believed it should inherit the star in the future. At the same time, this is for pragmatic reasons, and leads me to think it may well have been a combination of animals, plant life and familiars that had qualified for souls, although ancients are also a possibility. But the story never specifies, so we can but speculate. I'm agnostic as to the morality of the plan without knowing specifics, and furthermore, without knowing how they'd adjust their views on it if the full truth had been disclosed to them - they may have come to the view that waiting until Meteion was driven back and dissolving Zodiark to free the souls when it was safe to do so may have been the better course of action. He was deemed necessary at an absolute minimum to protect the star, but they didn't know that the root cause was a removable one.
I think it should also be noted that ensouled does not necessarily equal sentient in this context. Some carnivorous plants may be ensouled, but I would certainly never believe they were sentient. lol. People, and familiars with the ability to speak and possessing of self awareness, those are sentient.