Skype for Business Server requirements for Cloud PBX with on-premises PSTN connectivity

If i'm understanding this correctly, the only hard requirement for specific on prem roles being SfB 2015, is the Federation Edge and the next hop F/E pool as used by the Federation Edge.

So, if the above is true, my assumption is that what we're really talking about here is that the signalling traffic between on prem users and SfB Online must specifically route through SfB 2015 servers.

Media however can traverse Lync 2013 servers (Edge or Mediation), right?

This is important as if this is correct, one can simpy deploy a SfB Edge and SfB F/E pool into an existing on prem topology and use this as the Federation route. The new SfB pool does not even need to host any users or even have any conferencing or mediation workloads deployed.

Voice Routing policies will be configured and leverage PSTN Usgae records that direct calls to existing gateways/mediation servers (can be Lync 2013).

Really appreciate any confirmation you can provide that my understanding is correct (or not!).