I visited the place many years ago, when I was still a project engineer for te Cox Hobbies company.
Sanda-Kan was really something. It was a beehive of activity, located in Kowloon. The boss was a very nice man, a Chinese gentleman named Wai Shin Ting. Everyone called him, "Mister Ting". He passed away not too long ago.
The building was like 10 or 12 stories tall, but each floor had a 7-foot ceiling, because the woman working there, all Chinese, were very... short!
Each floor was buzzy with incredible activity, with injection molding machines, trimming and painting rooms, assembly rooms... and like a precision ballet, things were getting done, 6 days a week, 8 to 10 hours a day with a brief lunch break where the women would eat food they brought from home. And all were so kind, so proud of their work, and appreciative of the suggestions and comments the customers would make.
And they worked for so many American companies... Athearn, Atlas, K&B, Cox, just to name a few.
As far as your questions, no, the stickers were never sold separately, and the variants are few: there was a # 1 and a # 8, and maybe a # 5, and that was it!
Removing the roundels is not that easy depending on the degree of dryness of the glue. Best is to set the car in a warm place (but not in the sun or in an oven!!!) to soften the adhesive, then gently and slowly peel it off.
The Cox roundels are actually not too difficult, but the Monogram ones are really nasty.
At the end of the day, if the cars have been in a warm attic, they simply fall off, leaving a light glue residue on the bodies.
In any case, these roundels are rather easy to reproduce if they are damaged.