Actually now that I think about it: during the Gala episode we see a lot of upper crust earth ponies and unicorns, but aside from the wonderbolts, how many pegasi did we see?
Basically though, each of the races has sort of 'generic' jobs, but a lot of ponies end up doing other stuff. Like, Dash is on the weather patrol despite having little interest in weather; her passion lies in flying. Likewise, Pinkie works at a bakery but her actual passion is for party planning. Farming can be seen as fallback for Earth ponies because of their connection to the land, and weather working for Pegasi because they're the only ones who can. Unicorns don't really
have a fallback, though, since their big benefit is dexterity.
But yeah, it's not as rigidly divided as it seems. Fluttershy is perhaps the best example of the true nature. Ponies wind up doing what they want to do, regardless of creed. Rarity's boutique is possible due to dexterity but other creeds have similar jobs. Twilight is a scientist because of her interest; presumably other races could be just as studious. And so on, and so forth.