A 10-shot cylinder can be fitted to a 6-shot Model 17, provided the cylinder fits on the yoke (I think you'd need a -4 or later with the gas-ring on the cylinder, rather than on the yoke). You'd need a 10-shot hand (the bit of the lockwork that actually turns the cylinder, which is different for a 10-shot vs. the 6-shot). A 10-shot cylinder can also be fitted to a 6-shot Model 617, with the same requirement for a 10-shot hand.
The "standard" 17-8 had a 10-shot aluminum alloy cylinder (there's a different 17-8, the Heritage series, which had a 6-shot cylinder, glossy-blue finish, tapered barrel, and round-butt grip frame...this one is so different from the 10-shot version that some people say that it should've been called the 17-9). There was also a 617 with a 10-shot aluminum alloy cylinder, the 617-2 (although apparently later -2s started shipping with stainless steel cylinders).