I really think everyone Should have/Should be Determinant. [EPISODE 1 SPOILERS]
First off let me just say that the 1st episode was great and I can't wait for episode 2, but as for the topic I think that Marlon,Brody, and everyone else in the future should be Determinant, it would tell a better story for the player imo if they had a direct/indirect involvement in how every kid survives/dies in the story. Maybe you were able to talk Marlon down in the basement so he doesn't kill Brody, but she still tells everyone so the confrontment still happens and there is you can't call him down, he shoots Brody and if you do Brody lives, but AJ still might shoot Marlon based on the choices made earlier (kill the walkers, Fight Abel, and a 3rd choice they would of had to make for a tie breaker). That way it would really feel like your story and the choices you tell AJ could really change your story.
I do have a feeling that all the minor kids (everyone except Violet, Louis, and hopefully Rosie lol) will be Determinant just based off of the end screen for episode 1, but I still feel like they could have had this 1st episode have a little more freedom as far as the story goes.