We investigate synergism in mixed micelle formation and growth in two ionic/nonionic surfactant mixtures. In one mixture, the nonionic surfactant dodecyl hexa(ethylene oxide), or C12E6, is mixed with the ionic surfactant sodium dodecyl sulfate, or SDS. In the other mixture, C12E6 is mixed with sodium dodecyl hexa(ethylene oxide) sulfate, or SDE6S. The SDE6S head, which contains six ethylene oxide units and a sulfate group, "interpolates" between C12E6 and SDS and allows us to investigate in detail the effects of electrostatic and steric interactions between the surfactant heads on mixed micelle formation and growth. For both surfactant mixtures, me measure the critical micelle concentration (cmc) as a function of solution composition, and we use static light scattering to measure the weight-average mixed micelle aggregation number as a function of both solution composition and total surfactant concentration The analysis of the light scattering data accounts for both micelle growth and intermicellar interactions. We also infer the mixed micelle shape and hydration with combined static and dynamic light scattering measurements, and we qualitatively confirm the trends of micelle size as a function of solution composition with independent measurements of relative viscosity. The experimental mixture cmc and mixed micelle aggregation numbers compare well with those predicted by a recently developed molecular-thermodynamic theory of mixed micellization. The theory also permits a detailed analysis of the interplay of electrostatic and steric interactions affecting the formation and growth of mixed micelles in each surfactant mixture.