In a latest interview with Tales Series Producer Hideo Baba, he mentions that Sorey and Alisha from Tales of Zestiria are fixed party members in battle, possibly.
What does this mean? Baba-san mentions in the interview that the characters Sorey and/or Alisha (when she's actually in the party) are party members you have fixed in battle. With either Sorey or Alisha in battle, you can only actually change the Divine they are paired up with using the Partner Change feature. Can Sorey and Alisha be paired up? We don't know, but he mentions that it's mainly the Divine that you change around. Whether this is what he meant exactly is left for future clarifications.
The interview is from Bandai Namco Games JP's Side BN.
In other news, Tales of Zestiria will definitely be at Tokyo Game Show 2014 this September 18-21. Baba-san mentions they'll have new screenshots and images, a playable demo during the game show, and a stage event.