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I got some 10 speed veloce shifters I wanted to use with a shimano 8 speed bits
No problem. Just set the limiter screws of your reasonably old shimano mech at the ends of your 8 speed shimano or sram cassette, clamp shifter cable, take up the slack with the barrel adjuster and shift away. I've had the same setup for years, there's loads to read if you google 'shimergo', i.e.
http://www.cyclinguk.org/cyclists-library/components/transmission-gears/derailleur-gears/shimergo
http://www.cxmagazine.com/shimano-campagnolo-ergopower-compatibility
I got 10spd Veloce shifters to work with a full Shimano 8 speed drivetrain but the issues is that it put more strain on the shifter due to the difference in cable pull.