My name is MuseScript. I am a composer/Voice Actor/Fanfic writer. Here is a little example of my Voice Acting Skills. I can voice all of the Mane 6. But my best is Fluttershy. Here is a little something for the holidays: Carol of the Bells.
Let me explain my choices:

Rarity and Fluttershy as soprano 1s (top part):
*Rarity because she's a soprano in the ponytones (Filly Vanilli)
*Fluttershy because we have heard her sing very high (Same Episode)

Pinkie Pie and Rainbow Dash as soprano 2s (middle part):
*Pinkie Pie because it's comfortable in my range.
*Rainbow Dash (my favorite pony) for the same reason.

Twilight Sparkle and Applejack as altos (bottom part):
*Twilight Sparkle because the speaking voice sounds low.
*Applejack for the same reason.

Disclaimer: This is not for profit and no copyright infringement intended!

Happy Holidays!