@Larsenex 

Both will work with each other, how is a bit different in the back end.

So the Deco units are a mesh network that will broadcast one network name. As a device roams, they will jump to whichever node for the best connection. It is when you get the router where there will be some connection drops. That is because the mesh network is designed for seamless roaming, but when jumping to a network that is not mesh (A20), the device needs to connect the old fashion way. In doing so, you will get the monetary connection drop, but be back up and running. 

I assume stationary devices will do the majority of the streaming so I don't think that will be an issue. Should there be someone saying they were looking at youtube and it stalled for a bit, then it was probably the device going from the Deco to the A20 network.

With respect to the Deco nodes, I would set them as an access point, that way both can be wired to the A20. The mesh capabilities are still active in AP Mode.