With the increasing deployment of EV charging infrastructure, large numbers of power electronic converters are being integrated into distribution networks, making small-signal stability a key issue for converter-intensive charging facilities. In practical EV charging stations, grid-following (GFL) and grid-forming (GFM) converters may coexist and operate in parallel. Due to their different synchronization mechanisms and control structures, the stability of such mixed systems is affected not only by grid strength but also by EV charging power levels and power sharing among converters. This paper investigates the impedance characteristics and small-signal stability of three representative parallel configurations: all-GFL, all-GFM, and mixed GFL/GFM systems. Unified dq-domain impedance models are established for both converter types, and the critical short-circuit ratio (CSCR) boundaries are determined through eigenvalue-based analysis. The effects of charging power level and power-sharing ratio on the stability boundaries are further analyzed. The results show that the all-GFL configuration becomes unstable under low-SCR conditions due to phase-locked-loop-dominated dynamics, whereas the all-GFM configuration loses stability under high-SCR conditions due to reactive-power-loop dynamics. In contrast, the mixed GFL/GFM configuration exhibits both lower and upper stability boundaries and remains stable only within an intermediate SCR range. These findings reveal the combined influence of converter control type, grid strength, and charging-power operating conditions, providing guidance for the planning and stable operation of converter-intensive EV charging facilities.