Co₂Mn-based Heusler compounds offer a versatile, composition-tunable platform for magnonics and spintronics. Among them, the half-metallic Co₂MnSi is of particular interest for magnonics owing to its ultralow Gilbert damping, yet its weak magneto-optical response in the visible challenges optical probing such as Brillouin light scattering (BLS). We study the magneto-optical response of epitaxial Co₂MnX films (X = {AlₓSi₁₋ₓ, GaₓGe₁₋ₓ, Sn}) by magneto-optical Kerr effect (MOKE) spectroscopy and BLS. Angle-resolved MOKE resolves a significant, wavelength-dependent quadratic MOKE (QMOKE) only for Co₂MnSi, whereas Co₂MnAl, Co₂MnGa and Co₂MnSn respond dominantly linearly. Comparing BLS intensities of thermal magnons at two wavelengths, Co₂MnSi gives the weakest signal at 532 nm yet among the strongest at 457 nm, tracking the spectral dependence of its Kerr angle. These results emphasise the relation between the two magneto-optical techniques, guiding the choice of probing wavelength for Co₂Mn-based Heusler compounds.