Nice work overall! I like having a reference like this.
I did notice some areas that could use improvement.
Overall, the style seems solid. There are some proportionality issues, such as the legs being slightly too thin in the back views. Their eyes are the biggest issue, though. None of them change their style from the standard oval. This is okay with Applejack, Rainbow Dash and Pinkie Pie, who do actually use the oval design, but Twilight, Rarity and Fluttershy have very distinctive eye designs (Fluttershy especially).
Something else that needs to be addressed, stylistically, is the fact that in your reference sheet the manes occasionally switch sides. For instance, Fluttershy's mane goes right, right, left, left, left, with respect to the pony. The others have similar problems with this. Their manes also seem to switch proportions. This is very apparent with Pinkie Pie; in the rear 3/4 view (second from the end), her mane is noticeably taller than the middle image. Fluttershy has the same problem: the third and fourth images have noticeably flatter manes than the surrounding ones.
I realize you probably used show references, but as this is probably meant to be reference material for someone's pony drawing, certain adjustments need to be made.
I make blueprints as a hobby, so I'll offer a great trick to prevent inconsistencies like this: draw horizontal construction lines delineating key features, like the eyes, the top and bottom of the manes, the tail, and other important features. This can easily be done with layers, and you can make as many as you want. Then draw in the features, using these lines as a guide. Finally, erase the construction lines. This will definitely be of big help for references like this.