I'm trying to find a fic where Cadence/Cadance and Shining Armor/Armour discuss different spellings of a name. The end result being Kaydinss being joked about as the 'official' spelling of Cadance's name.

I think you're probably looking for On The Importance Of Spelling by Softy8088.

That's the one. (Now I just need the OTHER Twilight looping fic. It involves Spike getting turned into a pony. And she only 'loops' once...But that's in my faves somewhere. )

Thank you dubs.