  We present a general formalism that provides a systematic computation of the
linear and non-linear perturbations for an arbitrary number of cosmological
fluids in the early Universe going through various transitions, in particular
the decay of some species (such as a curvaton or a modulus). Using this
formalism, we revisit the question of isocurvature non-Gaussianities in the
mixed inflaton-curvaton scenario and show that one can obtain significant
non-Gaussianities dominated by the isocurvature mode while satisfying the
present constraints on the isocurvature contribution in the observed power
spectrum. We also study two-curvaton scenarios, taking into account the
production of dark matter, and investigate in which cases significant
non-Gaussianities can be produced.
