You don't want 7 unless you have a very tight ratio like the old corncob stuff back in the 5-6 speed era. The fewer gears the more spread they will have to do to get a wider range meaning more jumps between gears and will have you going back and forth between two gears that aren't ideal trying to find that middle gear that doesn't exist. That is what I did on my parts hybrid before I converted it to 9 speed a number of years ago after getting an XTR shifter gifted into my parts bin.