I have a mountain bike with a 7 speeds in on the back cog and 3 on the front. I recently got a Kickr Core. To use with my mountain bike could I use something like a Shimano 7 Speed Cassette, 12-28T, CS-HG200?
You'll need enough spacers to compensate for the width of the cassette and maybe readjust the rear derailleur. Your LBS mechanic will normally have a box of spacers, especially those from discarded 10 and 11 speed cassettes are convenient for fine adjustments.
I run a 6 speed cassette on a Tackx Flux with an early 1980s racebike. It works without any problems. The spacers go on the 'hub' ahead of the cassette. You may have to try out for the correct amount before the locknut sits perfectly.