The hybrid interconnection of grid-following converters (GFLCs) and grid-forming converters (GFMCs) has become a key feature of modern power systems, yet the risk of small-signal instability arising from their interactions remains insufficiently understood. To address this issue, this paper first establishes a full-order small-signal model of a GFLC and GFMC interconnected system. Parameter sensitivity analysis is then employed to reveal the subsynchronous oscillation modes induced by combined action of GFLCs and GFMCs, and to clarify the influence of system parameters on these modes. Subsequently, a single-input single-output (SISO) equivalent model of the interconnected system is derived based on the phase-locked loop (PLL)-dominated synchronization dynamics. By combining the complex torque coefficient method, a frequency-specific PLL damping enhancement method is designed to suppress subsynchronous oscillations caused by converter interactions, and a parameter design procedure for the proposed control strategy is also proposed. Finally, the correctness of the theoretical analysis and effectiveness of the suppression method are verified using a hardware-in-the-loop experimental platform.