To be honest, I think the reason why so many users are confused between cameo and guest role because there's no clear definition on what is guest role. The same thing happen with minor role and supporting/guest role, some users might think that this character is unimportant hence they add it as guest role, but others may not think the same.
In the guideline which Hessa created, it's only clearly explaining about the cameo, but not other roles (main, supporting and guest role).
So, I think it should be better if the admins here define what is main role, supporting role and guest role first. Once those 3 roles are defined clearly, we can continue to decide whether we can add other roles or not.
To be honest, I have added some roles here and there in this website but I added the role according to what I think, this character goes to this role and that character goes to that role, etc. In the end, I can only know whether it's correct or not after they have been merged by the staff here. But not all the role staffs here will think the same, so it's very vague.