8 speed shifter with 7 speed bike.

[DoctorRad]

Well, the distance between cogs isn't the same for 7 and 8 speed cassettes so you're relying on the floating jockey wheel to cope- if it can't then the gears will be slippy/noisy/generally nasty. Having said all that, I've not tried that combination, so..

This is the factor which turns the answer into 'it depends'. I've not run things that way around, but I did have one experience which might help someone else out.

I was running SunTour 8-speed thumbshifters with a Shimano 8-speed rear mech and a SRAM 8-speed cassette and chain. Wouldn't index. Swap the cassette out for an 8-speed Shimano and everything was fine.

I do remember that back in '93, the German magazine 'Bike' did a VERY comprehensive study into what worked with what. MBUK did something very similar sometime in 93-95 I think, but it wasn't quite as comprehensive.