Indeed, press releases are an allowed source for information.
You can read a little background on http://www.tvmaze.com/faq/16/episode-extras: In some shows, the credits make a distinction between different kinds of starring roles, like "Special Guest Star" or "Co-Star". If this information is available it can be entered, otherwise simply leave it at the default of "Guest Star".
So if the type of credit is specified - on-screen or in a press release - use it, otherwise simply leave it at "Guest Star". If the two of those conflict, the on-screen information takes precedence.