A detailed theoretical study of composite plasmonic waveguide structures is reported. Expressions for modal expansion coefficients, optical transmittance and surface intensity are presented and used to describe the behavior of dielectric channel waveguides containing a short gold-coated section. The superstrate refractive index is shown to control modal beating and modal attenuation in the gold-coated region leading to distinctive features in the surface intensity and device transmittance. The model presented allows detailed prediction of device performance, enabling improved design of highly sensitive miniature devices for evanescent refractometry and vibrational spectroscopy, and can be extended to the design and optimization of composite waveguides structures with nano-patterned overlayers.