Advanced Air Mobility (AAM) is expected to transform urban and regional transportation. However, its successful deployment hinges on robust infrastructure planning that balances operational efficiency with scalable growth. Current vertiport allocation models typically deliver single, static network solutions without considering phased growth or aircraft performance limitations. This gap is addressed by presenting a unified framework that generates a scalable vertiport allocation plan in conjunction with system-performance-based heterogeneous fleet sizing. First, potential vertiport networks are generated through a distance-based agglomerative clustering algorithm applied to the census-based synthetically generated demand. Second, the mean vertiport network is established, and undergoes an elimination procedure aimed at maximizing passenger km travelled, to establish phased growth of the vertiport network. In order to determine the optimal development of the fleet as the network grows, the framework employs an agent-based simulation and conducts a parameter sweep across all combinations of vertiport network size, fleet size and fleet composition. This framework is validated on a use case on the Republic of Ireland. The results demonstrate the ability of the framework to deliver a demand-driven phased network introduction and expansion strategy, while identifying the optimal fleet size and composition at each growth stage. This approach provides stakeholders with a replicable foundation for capital allocation and regulatory planning in emerging AAM markets.