Hey, It's That Voice!: Rina-chan appears as the voice of Nightmare Moon/Princess Luna. She also voices Cheerilee in Call of the Creepy.
A Bunch of Us: Hinted at in the videos (a letter from Princess Celestia in the first episode name-drops Batman Gambit), confirmed in the commentary (they mention how proud they are that this series and Sonic F both have articles here).
In the second episode commentary, when refering to Rina-chan, Mystic Pyro Freak slowly emphasizes "Hey, it's that voice."
A few of Mystic Pyro Freak's readings of Twilight's lines sounded a bit like Applejack, so the others suggested that she voice Applejack as well. Then, because Mystic Pyro Freak can't keep a consistent accent (by her own admission), she suggested the joke about Applejack's accent constantly changing.
In the initial script, Cyberlink wrote Fluttershy as a loudmouthed and obnoxious character, only to decide that this particular joke really didn't work. He couldn't think of a good way to make fun of Fluttershy—so instead he made a fourth-wall-breaking joke about being unable to bring himself to make fun of Fluttershy.