This is amazing, now all we need is the other main six as the other classes from FF1. Might I suggest this format? Fluttershy: White Mage Pinkie Pie: Monk Applejack: Fighter Rarity: Red Mage Rainbow Dash: Thief
That would go with the Monk's reflex passive skill where they can predict a move and dodge/counter it. But, if you want to get technical let's go with this. Two Canon facts that prove RD should be the thief. One: Pinkie is much to naive and innocent for something like thievery I mean if she can't even handle breaking a promise how can she be a PROFESSIONAL LIAR? Two: Pinkie Cannot control how or when her pinkie sense works and in those references it was due to those skills that she performed those tasks.
While RD may be the element of Loyalty, there is nothing against lying and cheating those you are not loyal to in her element while Pinkie couldn't steal due to it making somepony sad.