Previous studies indicated that intrinsic point defects play a crucial role for the density of states of ferromagnetic half-metals in the band gap region: at large concentrations, defect-derived bands might close the gap at the Fermi energy in the minority-spin channel. In this work, structural disorder in the Co and Mn sublattices of the full Heusler alloy Co_2−x Mn_1+x Si (−1 <= x <= 2) is investigated with a cluster expansion approach, parametrized using all-electron density functional theory calculations. By establishing two separate cluster expansions, one for the formation energy and one for the total spin moment, we are in a position to determine the stability of different configurations, to detect (also half-metallic) ground states and to extend the known Slater-Pauling rule for ideally stoichiometric Heusler alloys to nonstoichiometric, Mn-rich compositions. This enables us to identify potentially half-metallic structures in the Mn-rich region. With the help of Monte Carlo simulations based on the cluster expansion, we establish theoretically that Co_2−x Mn_1+x Si close to the stoichiometric composition ought to show a high degree of structural order in thermodynamic equilibrium. Hence, samples prepared with the correct stoichiometry should indeed be half-metallic after thermal annealing. Moreover, we predict that adding a small amount of Mn to stoichiometric Co_2MnSi allows suppression of the thermally activated formation of detrimental Co antisites. At Mn-rich compositions (x>1), the ordered ground-state structures predicted for zero temperature are found to be thermally unstable and to decompose into Co_2MnSi and Mn_3Si above room temperature.