The increasing penetration of converter-interfaced renewable generation and the declining share of synchronous generation reduce voltage and frequency support. This paper proposes a grid-forming (GFM) converter capacity ratio estimation method and a capability assessment framework for hybrid grid-following (GFL) and GFM converter systems at fixed total installed capacity. First, a model incorporating the principal control loops of both converter types is developed. The critical short-circuit ratio and the GFM converter terminal impedance are used to derive an analytical relationship between the GFM capacity ratio and the generalized short-circuit ratio. This relationship defines a continuous lower bound for the capacity ratio subject to small-signal stability and target grid strength constraints. The assessment framework uses normalized margins and a bottleneck index to evaluate basic support conditions, control parameter effects, and disturbance response performance. Generalized Nyquist analysis and time-domain simulations verify the method. Under the studied conditions, increasing the GFM capacity ratio increases equivalent grid strength and oscillation damping, reduces the peak power deviation, and shortens the settling time. Phase-locked loop parameters mainly affect synchronization dynamics and power oscillations, whereas virtual inertia and the damping coefficient primarily affect frequency support and power recovery. The method supports GFM capacity planning and control coordination.