
Honestly, I don't know.
The folks most people think of as the main cast of Star Trek, the show lists most of them as "Featured"
David McCallum and Leo G Carroll are both in the opening credits of Man From UNCLE, but they are listed as Co-Stars.
"Co-Star" was used at the end of credits as well to indicate guest star roles, but the term "Guest Star" wasn't really used much until more into the 1970s.
"With" and "And" also added emphasis to those actors and roles.
Like I said it's messy, and usually only makes sense to the people writing the contracts. :-)
For modern shows (say anything after 1980) the crediting is more like what we would expect. The Main Cast is listed in an opening credits roll, any guest stars are listed after the opening, and smaller guest roles are listed in the end roll credits. Even with the trend toward a splash screen open, there is still a separation of the main cast credits from the guest cast, either a distinct "Guest starring" break or you have the Main Cast credits, a "Created By" credit and then the bigger guest cast credits. I think our current policy deals with those shows just fine.
For shows pre-1980, they may have to be looked at on a case-by-case basis and information added to the edit wiki as to why the cast is listed like it is. (I guarantee there will be mass confusion if Doohan, Koenig, Nichols, and Takei are suddenly listed as guest cast, for example)
The caveat here, is I'm most familiar with how US shows work and I have an inkling on how the UK credits its actors. Beyond that, I have no idea if this is consistent for other countries.