I've said it once, and I'll say it again....I love this thread and the discussions brought up here!
I hate to say it but Theron did mess up pretty bad on Ziost. However, I feel it's more of a domino affect than him just sending in the Sixth Line blindly.
He believed.... ,
and as Xonell said, went according to the information given. He sent the Sixth Line, screwed up, knew it, tried to fix it. We also have Saresh, and infinite stupidity, to go along with this as well. Additional Republic forces would not have been on Ziost if it wasn't for her. Their blood is in her hands, not Theron's.
This almost feels like the Nar Shaddaa misson in Annihilation. Ziost was MUCH worse, but still, it's the same type of MMO. Theron believed he was going to prevent something, it blew up in his face and he was placed on leave.