Definitely a legit guess. Though how clearly the Master of Masters sees the future I think is still up in the air. Is he seeing something very clearly or is it a more of a jumbled mess with varying possibilities? Looking into the future can't be that simple.
Though while the Sixth Foreteller does sound like a candidate for the traitor, after I saw this video HMK posted in relation to the Rebirth theory I got some other ideas:
It also goes into details about the goat on the Keyblade as well. Such as what the goat and ram represent as a spirit animal.
As a supplementary I'll add a link for some info from the Chinese Zodiac on those born in the year of the goat (and doncha know it's almost Chinese New Year) : http://www.chinahighlights.com/travelguide/chinese-zodiac/goat.htm
I think someone may have mentioned this earlier in this post so forgive me if I sound a little bit like a parrot.
The possibility that the traitor is the Master of Masters. He tells his apprentices "Hey I think there's a traitor amongst you keep a sharp eye out!" which gets them paranoid and eventually turning on each other trying to figure out who the traitor is. This would get him off the suspect list because, he's someone the Foretellers would trust so he wouldn't go right to the top of the suspect list and him putting on the front of being concerned for his apprentices would help to get him off the suspect list.
The way I'd work in the Sixth Apprentice is that they are the red herring. Five out of six apprentices chosen to receive a sacred text? One was left out? Sounds like Foreteller number six would have a motive. Who ever our traitor is would realize by playing to the Foretellers' expectations not only does he get off the suspect list, he also gets a fall guy. And of one Foreteller were to strike down another Foreteller...that would be smashing Pandora's Box WIDE open.
Of course this could also still factor in the Sixth Foreteller being the traitor, witting or unwitting. In that case, they are a pawn who will eventually be disposed of in a variety of ways, examples: You fight them as a boss, the traitor behind the traitor pulls a "You have outlived your usefulness" on the poor sod, one of the Foretellers kills them, they are consumed by darkness and vanish, or they become a Xehanort vessel.