Right you are, Campagnolo 8 speed shifts the same as Shimano 7 speed -- all 7 speed, I think. So, if you have a Campy derailleur from before 2000 and Campy 8 speed shifters, it will shift perfectly over a 7 speed freewheel/cassette. You'll only have seven cogs, but that's ok cause you set the limits on the rear derailleur. About 2000 to 2001, Campagnolo went to 10 speed and changed their 9 speed stuff so that the rear derailleurs, as I understand it, move at a slightly different rate with respect to cable movement. So, Campy 10 speed and later 9 speed won't work properly with the earlier shifters. That's my understanding, never tried it. Also, most Campagnolo derailleurs are limited to about 26 teeth for largest rear cog. Long cage derailleurs, Racing Triple, for example, coud handle 28 to 30 teeth. This may be a concern, or not.
Unfortunately, the older Campy 8 speed stuff doesn't seem to go real cheap. I bought some Racing Triple derailleurs back in '97 and have recently seen similar stuff go on Ebay for mre than I paid then. Go figure.