This first summer league game as much as any convinces me they can play together, and to a certain degree what you call each won't matter.
HOWEVER;
-- defensively I believe Cousins continues to be the C, and Cauley Stein the new PF. WCS is so damn mobile, he can chase on the perimeter without any problem. be a bigger issue for Cuz. And on the other hand Cuz is enormously strong as a post defender, not to mention a great defensive rebounder and charge taker in there. So he should stay in the middle on defense, and Cauley Stein is the roving off ball length disrupting PFs.
-- offensively largely the opposite is the case, or I guess rather Cuz is the talent, and WCS is the garbage scorer. Cuz's post game would argue "C", but his overall in/out game is more versatile than WCS who just figures to clean up no matter what you call him.