For HG (Hyperglide) 7 & 8 speed cassettes, you need to use a 7.3mm pin (as opposed to other 8 speeds using 7.1). Both KMC and Shimano chains work well. I'm guessing the 6 speed chain you got doesn't work because there isn't a "shifting bridge" on the outer plate (6 speed chains are usually flat plates).

Shimano chains: HG40, HG50 (7,8 speed)
KMC: Z51, Z7, Z8