Should Abby, Luka and Chen be considered part of the "old" cast?
In later seasons, the series establishes a pretty clear split between the "old gang" and the new characters. This becomes especially evident in the last few episodes of the series when the "old" characters are called back for a final "throwback" get together. Carter, Benton, Lewis, Weaver, and Corday hang out together at the bar and later the hospital. Ross and Hathaway were featured in an earlier episode. Greene and Romano were brought back in flashbacks. Jeanie Boulet also came back in season 14.
I still feel that Abby and Luka could have been part of that "gang". Sure they were only introduced in season 6, and they basically stayed until the end so maybe a final comeback wasn't needed, but they were still central characters in the show when it still was the "old" show, and they had key interactions with nearly all the "old" characters. Kovac had a romantic relationship with Hathaway. Abby obviously had her time with Carter but she was also friends with Lewis and Weaver, and pretty close to Greene whom she saw as a mentor. They weren't part of the show when it became a sensation, but they still were part of its history.
And what about Chen? She was never THE star in the show, she was always kind of in the background but she was also technically one of the oldest characters in the show. She met all the old characters AND the new ones. She was possibly the closest non-romantic relationship Carter ever has in the show (maybe after Benton). She was there when he was a young inexperienced med student, she was there when he was a resident and got stabbed, she was there when he became a confident attending after Mark died. I feel she could have been included.