Because the term EXTRA or CAMEO is often misused, I searched for the right meaning of these and other words used in the credits.
I think these are the right ones.

EXTRA
Just a person on the screen of no importance to the story of the movie and who is not one of the following persons. Often misused in databases.
BITPLAYER
Somebody with a very small role, usely only in one scene or in one set. There are talking and silent bitplayers.
CAMEO
Very short role (silent or talking) played by a wellknown actor, or somebody important who's normally behind the camera. The actor must be clearly visible and there's only one appearence in the whole movie. In most cases the scene isn't important to the movie. Often misused in databases.
APPEARENCE
Some people call a very small role: an appearence.
GUEST APPEARENCE
Short role played by an welknown actor from another studio or an actor who normally plays in other kind of movies.
GUEST STAR
Leading, or important supporting role, played by an wellknown actor from another studio or an actor who normally plays in other kind of movies. Often misused, even by the studio's.
SUPPORTING ACTOR
Actor who plays a short or less important role.
LEADING ACTOR
Actor who plays a important role
STAR-STARRING
Leading actor with topbilling. One of the first names on the credits. The word starring is often misused.

About the extra-cameo topic, you are right.

For example, Sammo Hung does not do a cameo in "Big and Little Wong Tin Bar" (1962) because he was not famous back then... I have seen a lot of "cameos" which actually are bit parts o extra roles.

Some editors who add these terms doet not know their meaning.

Yes, "cameo" has historically been overused. I try to put something a bit more specific if I'm adding or updating roles. Like "thug" :p

How about a list of REAL cameo's?
I started with Tsui Hark
http://www.hkmdb.com/phpbb/viewtopic.php?p=44610#44610

By the way, I think that there are problems with crediting the 'fighters'.
They are no extra's because they are playing a part that is important to the film. They are not just moving or sitting around.
They are not bitplayers because they are not playing a real character.