  The streaming instability provides an efficient way of overcoming the growth
barriers in the initial stages of the planet formation process. Considering the
realistic case of a particle size distribution, the dynamics of the system is
altered compared to the outcome of single size models. In order to understand
the outcome of the multi-species streaming instability in detail, we perform a
large parameter study in terms of particle number, particle size distribution,
particle size range, initial metallicity and initial particle scale height. We
study vertically stratified systems and determine the metallicity threshold for
filament formation. We compare these with a system where the initial particle
distribution is unstratified and find that its evolution follows that of its
stratified counterpart. We find that change in particle number does not result
in significant variation in the efficiency and timing of filament formation. We
also see that there is no clear trend for how varying the size distribution in
combination with particle size range changes the outcome of the multi-species
streaming instability. Finally, we find that an initial metallicity of
$Z_{\rm{init}}=0.005$ and $Z_{\rm{init}}=0.01$ both result in similar critical
metallicity values for the start of filament formation. Our results show that
the inclusion of a particle size distribution into streaming instability
simulations, while changing the dynamics as compared to mono-disperse systems,
does not result in overall unfavorable conditions for solid growth. We
attribute the sub-dominant role of multiple species to the high-density
conditions in the midplane, conditions under which also linear stability
analysis predict little difference between single and multiple species.
