Here's the assigned instruments I have for the humane 7:

Sunset Shimmer - clarinet

• Reason: melodic sound

Sci-Twi (Twilight Sparkle) - clarinet

• Reason: melodic sound

Rainbow Dash - Trumpet

• Reasons: bright enthusiastic sound, and energetic air capacity

Pinkie Pie - Trombone

• Reasons: bright enthusiastic sound, sliding techniques (especially), and is definitely familiar with it

Applejack - Tuba/Sousaphone

• Reasons: familiar with bass notes, incredible strength to carry that much weight, and heavy air capacity

Fluttershy - Flute

• Reasons: light-weight, soft notes, and not much pressure of air capacity

Rarity - Colorguard

• Reasons: main instrument is piano, loves ballet dancing, and especially fabulous costumes 

Possible: Wallflower Blush - Flute

• Reason: melodic sound, doesn't want to be left out by almost everyone (again)