I just checked out my support. On their charts for the professional version it's written that the nonlinear geometrical capabilities are large deflection only for beams and shells and stress stiffening for beams, shells and solids. This seems to confirm your suspicion.
don´t know which version you are running, but in new 8.0 there is the MPC-Algorithm available in the contact wizard. Create a surface-to-surface contact by contact and target and set the contact behavior to bonded. It works fine and allows you to connect shell-to-shell (e.g different mesh size)...
