Small-angle scattering has been used to obtain detailed information on micellar contour lengths and persistence lengths, a measure of micellar flexibility, for mixed micelles of cetyltrimethylammonium 2,6-dichlorobenzoate and cetyltrimethylammonium chloride in water as a function of surfactant and salt concentrations and the relative amounts of the two counterions (the counterion inventory) at the micellar surface. Increasing amounts at the surface of the penetrating counterion of the pair, 2,6-dichlorobenzoate, lowers the 1D bending modulus of the micelles, making them more flexible. Micellar contour lengths exhibit a maximum with increasing amounts of a,2,6-dichlorobenzoate at the micellar surface. This is understood in terms of decreases in the micellar end-cap energies, E-c, at high concentrations of penetrating counterion. By decreasing the effective micellar surface charge density, penetrating counterions also cause decreased contributions from electrostatic repulsion, E-e, which retard micellar growth. At low ionic strengths, this results in more extensive micellar growth upon addition of sodium 2,6-dichlorobenzoate rather than NaCl.