  We report a comprehensive small-angle neutron scattering~(SANS) study of
Mn$_{1-x}$Fe$_{x}$Si at zero magnetic field. To delineate changes of
magneto-crystalline anisotropies (MCAs) from effects due to defects and
disorder, we recorded complementary susceptibility and specific heat data, and
investigated selected compositions of Mn$_{1-x}$Co$_{x}$Si. For all systems
studied the transition temperature and magnetic phase diagrams evolve
monotonically with composition consistent with literature. The SANS patterns of
the magnetic order recorded under zero-field cooling display strong changes of
the directions of the intensity maxima and smeared out intensity distributions
as a function of composition. We show that cubic MCAs account for the complex
evolution of the SANS patterns, where for increasing $x$ the character of the
MCAs shifts from terms that are fourth-order to terms that are sixth order in
spin--orbit coupling. The magnetic field dependence of the susceptibility and
SANS establishes that the helix reorientation as a function of magnetic field
for Fe- or Co-doped MnSi is dominated by pinning due to defects and disorder.
The presence of thermodynamic anomalies of the specific heat at the phase
boundaries of the skyrmion lattice phase in the doped samples and properties
observed in Mn$_{1-x}$Co$_{x}$Si establishes that the pinning due to defects
and disorder remains, however, weak and comparable to the field scale of the
helix reorientation. The observation that MCAs, that are sixth order in
spin-orbit coupling, play an important role for the spontaneous order in
Mn$_{1-x}$Fe$_{x}$Si and Mn$_{1-x}$Co$_{x}$Si, offering a fresh perspective for
a wide range of topics in cubic chiral magnets such as the generic magnetic
phase diagram, the morphology of topological spin textures, the
paramagnetic-to-helical transition, and quantum phase transitions.
