FWIW, I just build up a MTB for myself with a 7-speed rear because I already had the wheelset and some old school Deore 7S thumbshifters laying around. I'm using a 7S 12-32 SRAM cassette and 6/7/8 chain. The rest of the parts are actually "9 speed" (both derailers and Deore external bearing crankset) but everything works just fine together.