Within the same brand, you can't easily use different speed shifters/cassettes. Well you can with alternate cable mounting but that can be somewhat finicky .

However using different brand shifters with other brand cassettes does work due to the fact that one manufacturer might design their shifter to pull say 2.8mm of cable for their 10 speed cassette and rear derailleur but it also turns out that 2.8mm is what Shimano 8 speed shifters pull per click for their 8 speed systems. So if you use a Campy 10 sp shifter with a Shimano 8 speed cassette and rear derailleur, it pulls the same amount per click.