you dont implement a interface by making a variable. you gotta go to the bp which needs to implement the interface and on the details panel of that bp(if you dont see the details panel or cant find what im gonna talk about click on class defaults on top). and then there will be a list of implemented interface. you can add your own interface in that list.
Interfaces will also be inherited if you inherit a BP.
and i dont remember but i think you can get all class implementing a specific interface aswell and you can also use a reference to check whether it implements a interface or not. just search for it using the bp nodes.