Placing nanowire lasers on top of metallic substrates can shorten the duration of emitted pulses if coupling between lasing modes and Surface Plasmon Polaritons (SPPs) can be achieved. In this article, we perform time-resolved double-pump measurements of ZnO nanowires of varying diameters on aluminum substrates to investigate the relationship between nanowire diameter and emitted pulse duration. We find that nanowires with diameters below 190 nm emit pulses longer in duration than nanowires with a diameter above 190 nm. This suggests that the coupling between ZnO nanowire modes and SPPs of an aluminum substrate is suppressed for nanowires with diameters below 190 nm. We explain this behavior with the switching of the dominant lasing mode between azimuthal and linear polarization around a nanowire diameter of 190 nm. Thus, these findings highlight the importance of rigorous mode engineering in optimizing SPP coupling for low-loss metallic substrates, fostering future research on resonator design to achieve faster lasing dynamics.