Wheel is a 8, 9, 10 hub, can i place a 7 speed cassette?

You can use a 7 speed cassette, assuming it is the correct brand (Shimano is the standard for most cassettes/hubs)) to fit the splines on the freehub body. You will need a thick spacer to go behind the cassette because the 8,9,10 body is longer than the 7 speed one. A bike shop should have one for a few dollars. They used to be more common than they are now, but a good shop should still have some around. Less expensive wheels sometimes come with them already in place, as they are often used on older/cheaper 7 speed bikes. You will probably also have to adjust your rear derailleur.

If your talking about an old road bike, you may find that in addition to the spacer issue, the new wheel is a tight fit in the dropouts. Old stuff was 126mm wide and newer is 130mm. No 126's are available anymore, so you'll just have to live with it unless you do some surgery on the hub. Not an issue on mountain bikes though.

Hey Jacob, Short answer is yes. However... You'll need a spacer and you'll also need to have the rear derailuer adjusted. IF you're not sure how to do either of these tasks, take it in to the local bike shop. You can really mess up a hub and drive train if it isn't done properly.

NO. Why would you?