The ever-increasing adoption of power electronic converters for renewable integration and energy-saving applications has weaken grid and caused new types of small-signal stability issues, such as sub-/super-synchronous oscillations, resulting from the interaction between the fast-acting converter control and the power network. It is challenging to analyze the small-signal stability issues by grid strength assessment due to the complex interaction between many converter-interfaced generators (CIGs) and the power network in a multi-CIG system (MCIGS). This complexity is further increased when considering different non-rated operating conditions of each CIG, such as their actual power injections and terminal voltages, which manifest the heterogeneity in the MCIGS. Moreover, in such a heterogeneous MCIGS, it is difficult to characterize the small-signal stability boundary under non-rated operating conditions. To address the challenges, this paper derives generalized operational short-circuit ratio (gOSCR) and critical gOSCR (i.e., CgOSCR) by the small-signal stability analysis in such a heterogeneous MCIGS. Based on gOSCR and CgOSCR, a method is proposed for grid strength assessment to identify the small-signal stability issues in such a heterogeneous MCIGS. This proposed method can be used when CIGs are modeled by either black-box or white-box modeling in a MCIGS under non-rated operating conditions. More importantly, the proposed method can also be used to assess grid strength in terms of the static voltage stability in a MCIGS under non-rated operating conditions, since the static voltage stability issue is a special case of the small-signal stability issue with focus on zero frequency band for converter controls. The efficacy of the proposed method is demonstrated in the IEEE 39-bus system and a practical power system with a large-scale renewable integration.