They are separate products with no particular integration between them. As such, neither requires a license for interoperability with the other.
I suppose you could technically use ACS as an external identity store for ISE (although I've never seen anyone doing that). If you did so, no specific license would be required.
That said, ACS is a AAA server as is ISE.
ISE has always had its own RADIUS server which can act as the AAA server for network devices (in addition to its primary role as a network admission control system)
As of version 2.0, ISE has added TACACS+ support and can do almost all of the device administration functions most people do with ACS, If you want to use ISE as a TACACS server, it requires a device administration license in addition to any Base, Plus, Apex or Mobility license(s).
