Size and pump-fluence dependent ultrafast carrier dynamics of CdSeTe QDs are investigated using femtosecond pump-probe techniques operating at two different repetition rates: 1 kHz (low-repetition rate), and 76 MHz (high-repetition rate). With a low-repetition rate laser and 3.1 eV excitation photon energy, multiple exciton generation (MEG) is observed and the optical responses of alloyed QDs clearly show three components: a fast decay ascribed to carrier recombination, an intermediate component associated with MEG decay, and a slow decay associated with radiative Auger recombination. With a high-repetition rate laser and excitation photon energy resonant with band-edge energy, obvious coherent phonon oscillations are observed in 4 nm CdSeTe QDs due to impulsive stimulated Raman scattering. Open-aperture Z-scan measurement is used to clarify the size and pump-fluence dependence of optical nonlinearity under femtosecond laser excitation. With increasing laser power, an evolution from saturable absorption to reverse saturable absorption in CdSeTe QDs is observed. The transition process is analyzed using a phenomenological model based on nonlinear absorption coefficient and saturation intensity. These results indicate that CdSeTe QDs in a glass matrix are a class of materials for potential application in all-optical switching devices.