Oscillation instability is frequently induced by grid-following (GFL) converters under weak grid conditions. However, system stability can be significantly enhanced by the integration of a certain proportion of grid-forming (GFM) converters within renewable energy stations. Consequently, the capacity ratio of GFM to GFL converters is regarded as a critical factor governing system stability. To address this issue, an analytical capacity allocation model based on the critical short-circuit ratio (CSCR) is proposed. Initially, positive-and negative-sequence impedance models for both GFL and GFM converters are established to facilitate the analysis of sequence impedance characteristics. Subsequently, a three-port equivalent model of the hybrid grid-connected system is constructed, and the dominant control parameters affecting the interactive stability are accurately identified through sensitivity analysis. Furthermore, by integrating the generalized Nyquist stability criterion with the CSCR boundary conditions, an analytical model for the minimum GFM integration ratio, which strictly ensures the small-signal stability of the system, is rigorously derived. Finally, the validity of the theoretical analysis is verified through transient simulations. As demonstrated by the results, the critical GFM capacity can be accurately calculated by the proposed analytical model, whereby the system's impedance crossover frequency is successfully driven rightward into the stable region. Through this configuration, the phase margin of the system is significantly improved, and the broadband interactive instability under weak grid conditions is effectively suppressed.