Clarification about Epic Online Services Plugin


I am looking for some clarification regarding the Epic Online Services online subsystem plugin (EOS). I read that EOS enables cross-platform play but I am not sure I get the full picture.

My goal is to have a multiplayer game where users from Steam, Epic Store and potentially other platforms can play together instead of isolating the users of each platform.

What I am not sure about is this: When deploying my game to Steam, can I still ship it with the EOS plugin to retrieve the Steam user and Steam ticket id (need it in our backend) and to make ingame transactions on Steam? Or will I have to use the Steam online subsystem when shipping my product to Steam?

If I have to use the Steam online subsystem for this instead, will I still be able to join Sessions from a Steam client that have been created from EOS?

Will I have to compile my game once per online subsystem or is there a way to activate more than one online subsystem and switch between them at runtime?

Thanks in advance