The Ecosystem is currently in a scale up mode, where FIPs are integrating Account Aggrgeator modules with their system. To begin with, many FIPs have gone live with one or two Account Aggrgegators.
The link https://sahamati.org.in/fip-aa-mapping/  has the current FIP-AA mapping.
All FIPs are aligned to the principle of interoperability and are expected to integrate with all Account Aggregators over the next few months. ReBIT has specified standard protocols for APIs and security mechanisms, which ensures that a single implementation with one Account Aggregator is sufficient to be integrated with all Account Aggregators subject to a IP whitelisting and quick testing. Hence, there are no technical challenges to achieving interoperability.