07 August 2018
Are you looking for a conformal or a non-conformal interface between your solid and fluid regions? If you are looking to have a conformal mesh for your analysis, you can a 'Part' in DM with all your bodies. This will allow you to create a conformal mesh.
About your question on the interface selection, for CFX solver, you will have to create boundaries in CFX-Pre. All boundaries are added to Default Domain in CFX and you can create the necessary boundaries from there. If you have created a single part - CFX will recognize the interface automatically and will add it under 'Interfaces'.
I am also sharing part one of the four part video tutorial created on Youtube channel by CFD Ninja. I'd strongly recommend watching these videos.
Please let me know if I have completely misunderstood your question and elaborate on it.
I hope this helps.