The first thing I thought of the instant I saw this pic on EqD: "Oh look, a Red Mage Rarity!"
I click on it to see the source, and I'm immediately hit with the title: "Red Mage Rarity." As about as Exactly What it Says on the Tin as you can get. i can so see Rarity being a Red Mage just due to the fashion statement alone.
As far any others, I've always held that Twilight Sparkle is a Blue Mage. Always picking up spells just from observing others perform them, and putting her own twist to them (as well as making them horribly broken and overpowered in the process). She also comes up with a lot of her own, but the majority of the time I see her picking up a new spell, it's from her observing it being used by its original owner, or studying it from a book (which obviously had to come from someone before her). Thus, Blue Mage.
Oh, I'm well aware that the original six jobs in the first Final Fantasy are Warrior, Monk, White Mage, Black Mage, Red Mage, and Thief (yes, that ordering was intentional). Final Fantasy 11 paid homage to them by making them the only jobs available when you made a character (the rest were unlocked via quests). You've picked well the jobs for each of the Mane Six.
I'd say Twi would job change to Blue Mage eventually, seeing as it works very closely toward her modus operandi. Yes, I know Black Mage upgrades to Black Wizard in the original. I'm thinking outside that box (and apparently, so did Black Mage Evilwizardington).
Sorry for the lapse in response time, but I just had the following thought, if you want some inspiration. Here's the jobs I'd put the Mane six in:
Twilight Sparkle - Blue Mage (as I said in my previous post). Failing that, the next really obvious choice (blatantly obvious, in fact) would be Scholar.
Pinkie Pie - Bard. A really intense, really sugar-jacked, really pink Bard.
Rarity - Dancer. And her Dancer's attire would be absolutely FABULOUS!!!
Applejack - Paladin. An honest, stand-up job for an honest, stand-up pony.
Fluttershy - She'd most likely stay White Mage. I really can't see her in any other job. Except maybe Ranger (rather a violent job, but is usually paired up a lot with animals in the FF verse).
Rainbow Dash - Dragoon, definitely. Both take to the skies a LOT, both make use of said air time to do massive damage, and both don't give a damn when dragons show up (they charge in with gusto!). RD being The Lancer [link] of the group naturally almost makes this one too easy.
Carmen San Dia....no <.< red mage? Sorry, the red hat just clicked in mind mind as Carmen San Diago (untill I read the artist's comment) I admit, I'm not 100% on what a red mage looks like....anyway, Rarity looks very nice, you've done a great job making her look just like in the show ^_^ Awesome stuff *thumbs up*
Lol, what else is there to do 5 am in the morning? DRAW MOAR PONIES! >333
And now I'm thinking of 8-Bit Theater. So...I guess RD would be fighter? Fluttershy can be White Mage, and Spike can be black mage, and Luna can be Garland. Twilight Sparkle should be Thief, in my opinion, as she's most likely to understand sneaky contracts and is closely tied to royalty.
But Twi doesn't have anywhere near the right temperament! When's the last time she tried to backstab anypony? She'd probably make a fine Red Mage, though. For one thing, she is proficient in multiple schools of magic. For another, she's already come up with harebrained plans!
Twi: RD, you distract the parasprites! RD: AAAAAAAAAAAAAAAAAAAAAAAAAAAAH! Twi: And the rest of you, build an exact replica of Ponyville right over there. We have less than a minute!
Third, she strikes me as the most likely to resort to rules-lawyering.
Then again, maybe you selected her to be the Black mage due to her being insanely powerful and barely sane herself?
I guess I don't really know of anypony who'd make a good black mage. I figure Trixie would be a shoe-in for thief, though. Her arrogance alone makes it seem likely to me.