We present a general theoretical method for deriving effective susceptibilities for (non)linear optical scattering processes of arbitrary order using the reciprocity principle. This method allows us to formulate a generalized treatment of nonlinear optical scattering and deduce selection rules independent of the precise mechanism of light-matter interaction. We particularize this approach to second-order sum frequency scattering from an inhomogeneous medium and consider the limiting cases of small particle scattering, refractive index matched (Rayleigh-Gans-Debye) scattering, small refractive index contrast (Wentzel-Kramers-Brillouin) scattering and correlated scattering. We compare the derived expressions to experimental results of sum frequency scattering from monodisperse particles in suspension with varying sizes.