  The perturbative approach to structure formation has recently received a lot
of attention in the literature. In such setups the final predictions for
observables like the power spectrum is often derived under additional
approximations such as a simplified time dependence. Here we provide all-order
perturbative integral solutions for density and velocity fields in generalized
cosmologies, with a direct application to clustering quintessence. We go beyond
the standard results based on extending the EdS-like approximations. As an
illustrative example, we apply our findings to the calculation of the one-loop
power spectrum of density and momentum fields. We find corrections close to
$1\%$ in the mildly non-linear regime of $\Lambda$CDM cosmologies for the
density power spectrum, while in the case of the density-momentum power
spectrum effects can reach up to $1.5\%$ for $k\sim 0.2h/$Mpc.
