The crew isn’t only the pilots, it includes all flight personnel. Vueling schedules the whole team together; we recently had a delay due to a change of crew and it was the whole crew. Each crew can have changes between periods, but whenever a crew is set for a period they’re slotted as a team. Note that this is a company with relatively-short flights and a small list of possible planes: it’s actually easier to treat each team as a unit fitting a certain plane type than to schedule each individually.
I dated a first officer once. She was occasionally paired with a captain who she absolutely detested (and from her description sounded like like a horrible guy). Maybe different airlines work differently though depending on their size. There was no ability for her to be able to say “I can’t work with this person” although no doubt there must be some sort of process for problems between crew.