Latest Background Pony: Twilight definitely seems like she’d be the most Engineer oriented, intelligence is her thing and while she sets up buildings I can see her hanging back and reading a book. Although I like thinking of her as a Sniper solely because of the Sola Topi and Outback Intellectual items for him.
 
Pinkie Pie seems Heavy for the reasons you said, I mean the guy loves chocolate as well and has an umbrella hat.
 
As for Pyro, I always equated Derpy to that role. For the obvious being that Pyro has a googly eyes item, but also because of the latest Meet the video showing Pyro trying to do good while being a disaster for all those around it. Oh, and Pyro has a mailbox for a weapon.
 
Anyhoo they’re all up to personal interpretation. There’s wiggle room for a lot of these.

Zombie_Trotsky: Yeah, I agree with most of those castings. I’d make all the playable classes ponies, so basically the mane six plus three others (Derpy as Demoman?), while Spike is SAXTON HALE (I was inspired by one such picture).
 
I also like the idea of the each of the three “offensive,” “defensive,” and “support” classes being composed of one earth pony, one pegasi, and one unicorn each (so, say, a pegasus for Scout, an earth pony for Soldier, and a unicorn for Pyro). It might be a little overly anal, but I guess it’s kinda a fun little puzzle.

V Yeah the “new” fanon has Applejack as the Soldier, Twilight as the Engineer, and Zecora as the Sniper. Why Sniper? The Tribalman’s Shiv, the Razorback, “Jarate” being one of Zecora’s many potions…
 
Then again, there’s also some bits of fanon saying that Pinkie Pie is the Heavy (both big eaters, both genuinely friendly toward their teammates, both loud and gregarious) and Spike as the Pyro (because fire)

As much as people like to equate her to Engineer because of his Texan background and like the idea of apple dispensers, I just can’t say that I agree. She just wouldn’t let the fancy mathematics do the work for her.
 
I see her as more of a Soldier within the TF2 classes. She could take a hit, she can dish it out, and she can be speedy when the time calls for it. Most of all, she’d actually get herself dirty and into the fray, not just stay by the sidelines and setup a defense.

Well, going by personality would be hard seeing how all the TF2 classes are assholes who kill for a living, and do it with glee. So, I went by gameplay.  
AJ=Heavy (strongest of their groups)  
Fluttershy=Spy (not very intimidating at first, then becomes a pain)  
Pinkie=Demo (can go from place to place with ease)  
RD=Scout (duh)  
Rarity=Sniper (apparently hits pretty hard but doesn’t look like a bullet sponge by any stretch of the term)  
Twilight=Engineer(can teleport, does most work as indirect support)  
Spike=Medic(doesn’t do much attacking, but helps anyway with varying success (also can turn into something much more dangerous))  
Soldier and Pyro are open for suggestions.

Soldier, Heavy aas Applejack? I don’t see her as the type to yell obscenities as she mows down rows of enemies like weeds.
 
As for Demozecora. Well, she’s african, he’s black. That’s it really. Not perfect, but the best we’ve got with current charcaters.

Engineer is the nicest of the bunch (Pyro doesn’t really count). Whether AJ is in the mane 6 is another story.
 
But yes, Zecora as demoman leaves me with a bitter taste, since they have nothing in common. (implications)

Though I agree that AJ’s better off as a Soldier or Heavy, she does at least share a similar personality and accent with Engie.
 
People who make Zecora the Demoman have no such excuse.

“And if that don’t work? Use more apple.”

Well, most of Engi’s weaponry are created by his Grandfather, or came out of nowhere (like the Pomson).  
Which means he only perfected it, or just use it as it is.
 
And I think AJ does the same.

I can’t see Applejack as Engineer, because she’s not book smart.  
I’d imagine she’d be one of the stronger classes like Heavy. She does have the most power and doesn’t quite have a “normal” American accent.