Okay, now to the explanations why we choose those particular characters;
Nightmare Moon as Count von Krolock – despite the fact that King Sombra always fits into the role of a Vampire Count we didn't choose him for this role due to him already being featured as Count Dracula in Ink Potts concept artworks. Because ”Dance of the Vampires” and ”Dracula” have a slight connection with each other, me and Count-Monsparkle thought about choosing someone else for this role, and we came to the conclusion to turn Nightmare Moon into a male and make him the Vampire Count this time, Count von Krolock. This actually worked pretty well for us, and this choice actually created an interesting contrast between the colors of Krolock and Dracula.
Twilight Sparkle as Professor Abronsius – after long thinking and comparing with Dr. Whooves we decided that we should stick with Twilight anyway. She has a lot in common with Professor Abronsius; she loves books, logic and she fought Nightmare Moon, she is Fluttershy's friend + some other reasons.
Fluttershy as Alfred – Alfred himself is an unsure, kind and shy character, but when one of his friends, his true love or someone from his family is in danger he is always there to help and save them, no matter what. These characteristics fit Fluttershy's character really well, and we couldn't find a better character than her to ship with Pinkie Pie.
Pinkie Pie as Sarah – Both are kinda silly, want to have fun at the Ball, and have a pretty serious and caring family. Despite the fact that because of these choices I kinda ship Nightmare Moon and Pinkie Pie, but that's not the most important point with this character. Despite that everything fits really well and I'm satisfied with this choice.
Blueblood as Herbert – Blueblood was announced as Princess Celestia's nephew in season one, which leads to the theory that he could be a son of Princess Luna, or even Nightmare Moon, and that makes a slight connection between him and Nightmare Moon. Because of this connection the role of Count von Krolock's son has befallen on him. Also, Blueblood sounds kinda gayish, what is actually pretty much a true fact about this character.
Igneous Rock Pie as Chagal – The most straight explanation for this choice; Igneous Rock is Pinkie Pie's father, who cares about her and surely wants the best for her, and it would make no sense at all if we would choose someone else for the role of Sarah's father.