have you read the comics? Rarity has a friggin Applejack picture by her bedside. eeehhh but it's not official Hasbro I mean look at Equestria Girls where they're constantly... holding hands... it's as canon as a Y6 show can get.
Ahhh~~ Rarijack.. I lov' em! They just balance each other out so perfectly, they are both hardworking businessmares and help their friends. I also must say, that the complex character of Rarity needs a simple andlovely caring and brave couterpart as AJ is, so: Rarijack is NOT ONLY canon, but it makes so much sense in so many wonderful ways... And the ffs you can find on them are mostly beautiful ^^