I thought 5-6-7-8 all used the same chain width? single speed is thicker, 9 speed a little thinner, and 10 speed even thinner chain ?

I have used 9 speed derailleurs like early 2000 vintage Ultegra with 6-7 speed bikes no problem, with friction shifting, using standard 6-7-8 speed chains. worked /great/ in fact.