Interesting, I thought about making the storm king a daemon of tzeentch (due to his addiction to magic) But then, I took a closer look at his appearance, and I took tempest shadow into account. And suddenly I decided that the storm king would work better as a dark eldar archon, and tempest as a dark eldar wych...
I can see sunset as magnus, but sunset isn't as closely related to magic as moondancer
As for making tirek tzeentch, one: Color scheme, Two: tirek was the only villain to use brute strenght, rather then trickery and manipulation in his fight with twilight. Third: Khorne isn't directly opposed to the idea of using psychic powers: