It works for me and I'm also a total Mario nut. Though I'm trying to figure out whether the games would best be adapted as an action comedy or a slapstick comedy with small amounts of action. I'm seriously leaning towards slapstick due to the comical nature and clumsiness of Mario and Luigi.
I think this ship might as well be fully canonized to serve a purpose. Think about it's already semi-canon. All it would need is to be written into an episode. If Rarity is shown to coming to realize her own feelings for Spike, it could set up the lesson that every girl is a prince or princess to the right guy, but sometimes "Prince Charming" isn't who you'd have expected him to be. Sometimes you spend your days your days looking for him when he has been right in front of you the whole time. It could even end with them sharing a mildly romantic moment together.
Garble's an asshole and is the complete opposite of Luigi Mario. Luigi is cowardly but kind and loyal. He is good natured and devoted to his friends. He's extremely shy and timid and will run and hide at first sign of danger unless his friends are in trouble. Now who does that sound like? Fluttershy!