given what actually happened when reven and malak fought the emperor. The emperor would just turn all 5 of them into his personal puppets
Maybe, but I don't think it'd be as simple as that given that Darth Nihilus' Force Drain could weaken Darth Vitiate. Also, Darth Sion is nigh invincible, so he could be a nuisance. As I said, it'd be an interesting bout if these 5 teamed up to try and stop the Sith Emperor instead of doing what they done in KOTOR 1 and 2.