[Q]

If I think about it, i can't recall that I ever saw the AI demand surrender! Did anybody else??
About your second question: I never choose the mechanoid trait, because I think it is rather easy to research the medical technology that protects you against plagues. But you must give the AI a ship design with medical bays, otherwise he will be helpless against plagues even if he researched the medical technology. But if the AI has a ship with a medical bay, he will send it to the plagued colony.