We present the first detailed particle-in-cell simulation studies of stimulated Brillouin scattering (SBS) in an underdense plasma composed of multi-ion species. In particular, collisionless nonlinear saturation mechanisms are well modeled, and are found to depend critically on the plasma composition. The effect of a self-consistently evolved non-Maxwellian ion distribution can be observed in detail. He, H, C{sub 5}H{sub 12}, C{sub 5}D{sub 12}, CO{sub 2}, and various mixtures of these gases are studied in these simulations. The reflectivities due to the SBS instability show many of the trends seen in experiments with these gases.