We have already shown that binary Coulomb compounds in disordered states are stabilized against phase separation in a minor ion rich region as charge asymmetry is enhanced; a stable phase appears approximately beyond the charge ratio R-Z = 6. In this paper, the mechanical properties of the stable compounds are elucidated through extensive calculations of the Coulomb energy increments due to virtual deformations. The compounds turn out to be much more isotropic than the mixed pure crystals in the linear response regime. And their effective elastic constants obtained by averaging over directions are well expressible in terms of those of the pure crystals. The compounds, however, show fragile behaviour at the degree of deformation where the pure crystal still retains its elasticity.