Since this is a parody/mashup of My Little Pony and Walt Disney's "The Sorcerer's Apprentice", I believe it falls under "Fair Use" and it's okay to sell it as a print and t-shirt, etc. You can purchase the t-shirt here: www.teepublic.com/show/372646-…
So...are you a mare or a mouse?
You've captured Twilight extremely well here. I think her head tapers to the nose a bit more than on the show, which adds to your style for her. The hat and the robe fit her well. And your coloring of the whole piece has been nicely done. I also really like the play on words with your title.
I noticed you had a more complete version of this with a background and the villain from the movie as Chernabog. I'm a bit surprised you didn't share that version. I favorited that one too.
I think my two problems with this piece are related to the stroke outline around Twilight, and the way she interacts with the rock. I dunno, it looks like you've tried to merge Twilight and the rock on the same layer by chopping out her tail and the ends of her feet, rather than placing her tail on a top layer, and her body on a bottom layer, with the rock in between. That would have allowed you to have everything together without having to carefully erase bits and pieces.
I also think the stroke outline, at least in this version of the image, should have encompassed the rock as well, not just Twilight's body and tail up to the edge of the rock.
If you ever decide to, I'd really like to see a version of the Greek scene with the other unicorns and pegasi represented by MLP characters. Last time I watched Fantasia I really thought someone should do that, since I feel like that Greek sequence is what may have inspired the My Little Pony toys in the first place.