Linear and nonlinear viscoelastic properties were examined for aqueous suspensions of monodisperse poly(methyl methacrylate-co-styrene) (MS) particles having the radius a(0) = 45 nm and the volume fractions phi = 0.428-0.448. These particles had surface charges and the resulting electrostatic surface layer (electric double layer) had a thickness of t(s) = 5.7 nm. At low frequencies in the linear viscoelastic regime, the MS particles behaved approximately as the Brownian hard particles having an effective radius a(eff) = a(0) + t(s), and the dependence of their zero-shear viscosity eta(0) on an effective volume fraction phi(eff) (= {a(eff)/a(0)}(3)phi) agreed with the phi dependence of eta(0) of ideal hard-core silica suspensions. In a range of phi(eff) < 0.63, this phi(eff) dependence was well described by the Brady theory. However, the phi(eff) dependence of the high-frequency plateau modulus was weaker and the terminal relaxation mode distribution was narrower for the MS suspensions than for the hard-core suspensions. This result suggested that the electrostatic surface layer of the MS particles was soft and penetrable (at high frequencies). In fact, this "softness" was more clearly observed in the nonlinear regime: the nonlinear damping against step strain was weaker and the thinning under steady shear was less significant for the MS suspension than for the hard-core silica suspensions having the same phi(eff). These weaker nonlinearities of the concentrated MS particles with phi(eff) similar to 0.63 (maximum volume fraction for random packing) suggested that the surface layers of those particles were mutually penetrating to provide the particles with a rather large mobility.