Why doesn't Kuzco have a food taster? It's because he thinks the whole world adores him. The thought that someone might dislike him, let alone try to kill him, never even crossed his mind.
When Yzma's goons all get turned into a random assortment of animals, none of them seem nearly as alarmed as you'd expect... save one guy, who asks to be sent home early on the grounds that he is now a cow (Yzma allows it). Why is he the only one showing any concern? Well, despite what pop culture would have you believe, cows are exclusively female. Poor guy didn't just have his species changed, his biological sex was changed as well, which is disturbing and unexpected even by the standards of someone used to Yzma's transformation antics.