Who Zoro and Sanji fight will depend on how Oda Portrays Luffy fighting a commander currently. Oda doesnt like downplaying Luffy, which is why he had Luffy defeat one of the cp9 really quickly before one of the strawhats did. Because Luffy previously got destroyed by cp9. So he had to show Luffy was much stronger, for it to make sense why the other strawhats were capable of defeating cp9 members. If Luffy manages to defeat a commander similarly to how Blueno was taken out. Then I think that would tell us what Odas plans are. Nobody aside from Luffy is having a warm up fight in my personal opinion. Whoever the characters fight will be their final fight for the arc. For Sanji the bare minimum is a Tobi Roppo, For Zoro the bare minimum is a Tobi Roppo and joining the fight against Kaido. Regardless of what they do, we know for a fact the strawhats who will shine the most are Zoro, Sanji, Nami, Chopper and Usopp. Even Toei knows lol.