The magnetic Prandtl number ($Pm$), defined as the ratio of kinematic viscosity to magnetic diffusivity, plays an important role in determining the dynamical response of electrically conducting fluids to an imposed magnetic field. Nevertheless, its influence on convective onset and, in particular, on the preferred roll orientation under an inclined magnetic field remains incompletely understood. In this study, we investigate the onset of rotating magnetoconvection in a horizontally extended layer of electrically conducting fluid subjected to vertical rotation and a uniform magnetic field inclined in the $yz$-plane. We perform a linear stability analysis of the motionless conduction state using a staggered-grid Chebyshev spectral collocation method over wide ranges of Taylor number ($Ta\in [0,5\times {10}^4]$), Chandrasekhar number ($Q\in [0,2\times {10}^3]$), magnetic Prandtl number ($Pm\in (0,10]$), and thermal Prandtl number ($Pr\in (0,4]$). For stationary onset under a horizontal imposed magnetic field, increasing rotation drives a parallel $\to$ oblique $\to$ cross roll sequence, while tilting the field toward the vertical progressively contracts the oblique-roll regime. At overstable onset, low-$Pr$ fluids, particularly in the low-$Pm$ regime relevant to liquid metals, preferentially select parallel rolls. For cross and oblique rolls, finite $Pm$ modifies the magnitude and phase of the induced magnetic field and the associated Lorentz force contribution to the stability balance, allowing their overstable thresholds to fall below competing modes. Within the investigated parameter ranges, increasing $Q$ or $Ta$ generally enlarges the overstable region. For horizontal and moderately inclined fields, increasing $Pm$ is associated with an expansion of the cross and oblique roll regimes, whereas tilting the imposed field toward the vertical suppresses overstability and favors stationary onset.