Yes. The captive portal will work with your current network setup. Simply connect the Mini so it sits between your LAN (internet source) and your existing network. The Mini will act as a bridge and display the captive portal when someone connects to your current access points.
Additionally, as long as the Mini/Cube is on the same network as the Edge Extender(s), it will discover the Edge Extender(s) and broadcast your captive portal. The captive portal will work with Ubiquity, Cisco, and so many others.
Routie provides dedicated phone support to enterprise customers. We are happy to get our technicians on the phone with you to help answer any specific questions related to your setup.