I agree with you both. Alan Rickman is perfect for the part of Snape, and always makes an excellent bad guy.
Rickman's best roles are when he is the smartest person in the room and has to endure the stupidity around him. He makes a hilarious comic villain.
He did a great "Euro trash" part in Die Hard.
Miranda Richardson is not the first thought I had to play Skeeter but I bet she'll be great in the role.