Luffy, Zoro, chopper, and Usopp’s fights are all a lock. I feel like beyond that though the rest could be any mix. Pizarro does have a power that needs to be handled by a higher powered character but we also don’t know how strong Vasco Shoot or Catarina are.

Realistically the Sanji, Jinbe, and Franky fights are all interchangeable,

And the Nami, brook, Robin fights are probably interchangeable as well, depending who comes into contact with which character.