EXCLUSIVE: Alex Schack, who has spent the past seven years at Slate PR most recently as VP, is moving to Narrative as the entertainment public relations company continues to grow its staff. Schack’s roster of clients at Slate included Bryce Dallas Howard, Madeline Brewer, Katherine McNamara and Dove Cameron among many others, and all are expected to follow her, we hear. Narrative was formed in December by Liz Mahoney, Megan Moss Pachon and Bryna Rifkin (from ID PR) and…