Nearly all or maybe all modern EPA woodstoves use a 6" pipe from stove to chimney. Many will work fine venting into an 8" flue, but there are other factors involved: chimney height, whether it's an internal or external chimney, etc.

What you can't do, because of building codes, insurance regulations, and your own safety is merely stick a 6" pipe into the existing 8'" chimney. The chimney needs to be inspected and quite possibly lined using an insulated stainless steel or poured masonry liner. A blockoff plate also is required below where the stovepipe enters the chimney.