Pope Innocent's quote is fallible, because he was speaking as a fallible pope. The pope is only infallible when he speaks with his APOSTOLIC authority, on faith and morals, when he intends to teach and bind the whole church to believe an article of faith. Outside of this, he speaks as a fallible bishop.
The authority of "augustine and ambrose" can't be compared to the authority of the Council of Florence/Trent. Also, as has been pointed out, St Augustine waffled on his opinion in this matter.