Max,
Not to bash nor boast but just a note, I can beat ANY single-stack AI army with 6 (scottish) border horse, including mongol and timurids on any dificulty setting. I have dial-up internet so am unable to play online but have played via LAN and can tell you without question I am no match for the elite onliners. Put simply, the ability to defeat the AI, no matter the odds, has no relevence in the MP world.
I dont doubt you can quickly become a contender, but I advise you not go into online play assuming your the best as some players are likely to take it personally and you will not take lossing very well. On that note, is there an undefeated online player?