This paper presents an approach for developing two-dimensional plate theories within the Unified Formulation framework in quasi-static nonlinear analysis. In nonlinear structural analysis, achieving accurate stress predictions with computationally efficient models is particularly valuable, as these simulations are inherently computationally demanding. The proposed methodology enables each displacement component to be represented by an independent expansion function, permitting the integration of different kinematic theories within a single model. Furthermore, it allows for the combined use of Equivalent Single Layer and Layer-Wise approaches. The structural model is implemented in a finite element context using an enhanced version of the Unified Formulation. The governing equations are linearized using the Newton–Raphson method, and the Crisfield arc-length method is employed. The accuracy of the proposed models is validated against literature data for isotropic and composite plates, focusing on large-deflection and post-buckling responses.