I'm definitely suspicious about us having mandetory party members for the main story and then optional for grinding after we reach a way point.
Everyone moves to a fixed point, but before that the party was chosen for us. There's specific cut scenes with the missing party members that wouldn't work if we chose the others.
But, once we hit a way point it doesn't matter who we pick. We can go back and level grind just like OG.
Because if you go back into the lab there's 3 failed experiments waiting. If you kill them and go back to Chadley, everyone moves to their fixed point. If you go back to the lab again, the same 3 failed experiments have respawned.
Basically, we're getting fixed party members for the entire story and only get to swap them out if we wanna go back through a dungeon to level up because it doesn't matter who you take.
We're getting canon everything this time. The people with you are the people meant to be with you for that point in the story.