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@rodabod
I wouldn't be too concerned about running a 9-speed chain on an 8-speed set up. It's a bit like running an 8-speed chain on an older 5-speed setup which generally works fine.
Anyway, back to the original question of compatibility, things only got complicated when they first brought out 9-speed as there was a transition period where the cable pull changed. The way to check which type of mech you have (old ratio versus new) is to see what style of b-adjustment it has. The tension screw moved you see.
If you have mated the correct shifter with the correct mech and it still doesn't shift well (and the hanger is roughly straight, then you should check the mech does not have too much play in either the parallelogram or the jockey wheels. The parallelogram should only move the jockey cage back and forth and not wobble at all against the hanger mount. The upper jockey wheel should have lateral movement, but have only a slight amount of play to allow the wheel to angle side to side. Also make sure the wheels are the correct way around; upper and lower! I've seen them swapped before when the upper has had so much play it's become useless...