Drupal 7 default Forum styling for custom content type?
By 'have the same styling', I'm assuming what you mean is have the same structure and functionality as the Drupal core forum (i.e. threaded discussions). You can always modify the content template to match the same layout and css styling of the core forum module but just enabling comments won't give you the same functionality as forums. The forums module uses comments but adds another layer of management on top which is where you get your threaded discussion. A comments-only content-type would also not integrate with forum add-ons like Advanced Forum. 

You're best option is to create two distinct forums and then (as @frazras notes) use Forum Access module to control access based on your criteria.