I never heard anything about a Zasalamel model in the files, but there is no reason to assume they'd use another main character's model besides story implications. They could have used just a generic one. With Raphael, on the other hand, he not only shares his model with custom Nightmare, but also shares his voice with Graf Dumas and Nightmare, things he wouldn't if there weren't story implications, they did hide Yoshimitsu's face and made adaptations to Nightmare's demonic arm, as such, its not like laziness would lead to something as copying a main character model.