As mentioned above, I also use to do Z at the start of all my background characters, but really adapted to my new style and doubt I will ever change it.
I would just simply call them EXTRA1 and EXTRA2.
EXTRA1, EXTRA2, etc… for the ones that don’t talk, or RANDOMGIRL1 and RANDOMBOY2.
However, if they actually have speaking lines, I give them names, or names related to their job, like SALESWOMAN, WAITER, etc.
BGMAN1, BGWOMAN3, e.t.c
I either do:
F1/M1
EXW1/EXM1
I do like ZGIRL1 and ZBOY2 for scenes with bg characters. I use X Y Z letters or a combination depending on the scene and just change the display name if the characters talk. I use XYZ so they don’t get in the way of my main characters when I look through the animations library.