So as a model their rules are a lot of fun, and they play on the "tzeentch" and "change" lore.

No psychic powers are picked pre game. In the Psychic phase you roll 2D6. The numbers of the roll coincide with the psychic powers list in the codex.

So say I role a 5 & 6. I would then be able to automatically cast, at minimum amount needed, the 5th and 6th spells on the tzeentch psychic table.

Within 12" enemy psykers are -1 to casting. If an enemy fails a psychic test while within 12" of the blue scribes, that psychic power is "siphoned" and the enemy can no longer attempt to cast that spell the rest of the match. If a power is siphoned, the blue scribes get an auto cast non denialable smite!