serval questions of heat transfer in my project

Hello
As the figure shows, I am using the fllowing models
(Heat Transfer>Conjugate Heat Transfer>Laminar Flow
Stationary situation
Surface-to-Surface Radiation), and try to find the temperature distribution of the air and the spheres.
I simplified the original object into two spheres,since its shape was very complex. The latter sphere has a boundary heat source.
I have some questions to solve:
1.Since the object was simplified, so the real convective area was changed, i need to modify the convective heat transfer coefficient, but I can't find the modify interface in comsol.
2.The Surface-to-Surface Radiation was considered, and the model is periodic, so I need to define the periodic boundaries as perfect mirrors (the default insulated conditions are just fine to the convective heat transfer and conduction heat transfer)， but I can't find the interface in comsol,too.
3. Since the original object was simplified into two spheres, so the heat conduction was incorrect，because the spheres are not contacted.I want to find the ht.gardT between the spheres and use the Fourier's law of heat conduction to define extra heat source on spheres, is it right?
thank you