So...

Dash = Assassin/Thief

Rarity = Mesmer

Pinkie = Engineer (Hell, mortar elite skill or any of the turrets arguably function as the party cannon)

Applejack = Guardian (doesn't matter she's an earth pony, she seems likeshe would be the supporting tank)

Fluttershy = Ranger

Twilight = Necro (more likely elementalist, but I could see her delving into the dark arts. When they're apparently not so dark)