Did Israel already have organization before Jethro's advice about rulers?
Yes. Israel already had organization before Jethro's advice. They came out of Egypt with their armies and already possessed a relatively good system by tribes. Jethro's counsel about rulers over thousands, hundreds, and fifties was given to a people who were not merely a disorganized rabble.
Les explains that Israel was not a disorganized mob when Jethro gave his counsel. He points out that when they came out of Egypt they already had organization; they came out with their armies, and they already had a relatively good system by tribes. That existing structure matters because Jethro's advice concerned appointing able men—men who fear God, men of truth, hating covetousness—as rulers of thousands, hundreds, and fifties. Les also stresses that the chapter never says “and the Lord said”; the Lord is out of the picture, and this is a man speaking in the energy of the flesh, even though the advice sounds logical and is not basically wrong. Moses nevertheless harkened to his father-in-law's voice and did all that he said, beginning to pick out men. So the counsel was layered onto a people who already had tribal and military organization, not created out of nothing.