  This paper aims at studying the reliability of a few frequently raised but
not proven arguments for the modeling of cold gas clouds embedded in or moving
through a hot plasma and at sensitizing modelers to a more careful
consideration of unavoidable acting physical processes and their relevance. At
first, by numerical simulations we demonstrate the growing effect of
self-gravity on interstellar clouds and, by this, moreover argue against their
initial setup as homogeneous. We apply the adaptive-mesh refinement code {\sc
Flash} with extensions to metal-dependent radiative cooling and external
heating of the gas, self-gravity, mass diffusion, and semi-analytic
dissociation of molecules and ionization of atoms. We show that the criterion
of Jeans mass or Bonnor-Ebert mass, respectively, provides only a sufficient
but not a necessary condition for self-gravity to be effective, because even
low-mass clouds are affected on reasonable dynamical timescales. The second
part of this paper is dedicated to analytically study the reduction of heat
conduction by a magnetic dipole field.We demonstrate that in this
configuration, the effective heat flow, i.e. integrated over the cloud surface,
is suppressed by only $32$ per cent by magnetic fields in energy equipartition
and still insignificantly for even higher field strengths.
