Abstract: With the large-scale integration of renewable energy sources and grid-following power electronic devices, the voltage support strength of power systems decreases dramatically, posing threats to the secure and stable operation of the new type power system. In the scenario where homogeneous power electronic devices are integrated into the system, the so-called generalized short-circuit ratio (gSCR) of the power network and the critical short-circuit ratio (SCR₀) of the device/stations can be used to quantitatively evaluate the voltage support strength of the system. Such a method is highly related to the system stability. In the scenario where heterogeneous power electronic devices (exhibiting weakly different dynamics) are integrated into the system, the voltage support strength can be quantified by the first-order approximation of gSCR and SCR₀, utilizing the special characteristics of the devices and the power grid. However, there is still a lack of unified derivation principles and calculation methods to handle the heterogeneous cases. To this end, this paper focuses on quantifying the voltage support strength of the system with respect to small-signal stability. We found that the multi-infeed system can be approximately decoupled into multiple low-dimensional systems. On this basis, we propose the concepts of eigen-subsystem and its calculation method, and further interpret their physical meaning. Based on the concept of eigen-subsystems, the generalized derivation principle and calculation methods of gSCR and SCR₀ are proposed. Additionally, the specific calculation methods for these metrics are provided when considering grid-following converters under non-rated operating conditions, reverse active power outputs, and grid-forming devices. Finally, the effectiveness of the principles and methods is verified in numerical case studies.