The microstructure and flow properties of bimodal mixtures of charge-stabilized colloidal particles are investigated. Using small-angle neutron scattering (SANS), an initially ordered lattice of monodisperse 220 nm diameter particles at a volume fraction near the order/disorder phase transformation boundary is observed to maintain long range orientational order with the addition of a small number density of 54 nm diameter particles. As the number density of these "defects" is increased, the colloidal crystal loses long range order abruptly. Changes in the rheological properties of these suspensions are correlated with changes in microstructure. The initially ordered lattice is a viscoelastic solid, but with the addition of a sufficient number density of "defects", the suspension behaves as a purely viscous fluid. The changes in rheology and microstructure are not observed by simple dilution of the 220 nm particles.