Density functional calculations with simulated annealing have been performed for singly charged mixed silicon-carbon cluster anions SinCm- and for neutral SinCm clusters with up to eight atoms. The calculations predict the existence of a number of previously unknown isomers and identify two classes of anion structure : carbon-rich (chainlike) and silicon-rich (three-dimensional), with pronounced differences in the vertical detachment energies (VDE, transitions to states of the neutral clusters). The results provide insight into recent photoelectron detachment measurements on the anions.