You cannot combine the U6-Mesh WiFi 6 (4800 Mbps) directly with your existing AmpliFi Alien Router + Meshpoint to operate a unified mesh network. The two systems are not compatible and cannot be controlled together as a mesh system.

You can only operate the U6-Mesh WiFi 6 (4800 Mbps) as a stand-alone access point if you integrate it into the network separately, but not as part of the AmpliFi Mesh system.

For a standardised mesh network, you have to stick with AmpliFi or switch completely to a UniFi system.