A character with a Mark of Chaos cannot join a unit that has a different Mark of Chaos. A character with a Mark of Chaos cannot join a unit that has already been joined by a character that has a different Mark of Chaos.
A Daemon of Tzeentch has Hatred (Daemons of Nurgle) and Ward save (6+). Wizards with the Daemon of Tzeentch upgrade can also re-roll channelling results of 1.