We extend our previous study of low-frequency waves and instabilities in stratified, gyrotropic (with a thermal pressure characterized by P$perp$ ≠ P$parallel$), two-component plasmas to multicomponent plasmas. We then apply our theoretical findings to the plasma transport driven by the interchange instability in the Io torus. Our derivation starts with the correct calculation, from the Vlasov equation, of the contributions from all plasma components to the thermal pressure perturbations of the whole plasma. By incorporating the expressions of the thermal pressure perturbations into the classical system of fluid and electromagnetic equations, we are able to derive the dispersion relation as well as the local stability criterion of low-frequency modes in stratified, gyrotropic, multicomponent plasmas. Much of our discussion is devoted to the modes on which stratification has a significant impact, i.e., the so-called quasi-interchange modes. In a second step, we apply the stability criteria obtained for the quasi-interchange modes to realistic equatorial distributions of plasma and magnetic field in the Io torus based on the most recent available observational data. We perform this application in order to gain new insights into plasma transport in the torus and compare our results with previous studies on this subject.