Assuming that I have two sets of 7-speed shifters and one wheel with an eight speed cassette, can I swap the 8 for a 7 or is the spindle too large for that? Or should I just run the 7 smallest gears until I can fins a set of 8 speed shifters?

(purple Cannondale M-500. I'm not sure of the year but all but one year of M-500 have 7 speeds and the sifter only clicks 6 times so I am am guessing that I do NOT have a '98)