Q1) Can I install and use Couchbase Server 6.0.0 on Windows 7 and 8?
Q2) If I cannot, then can I use Sync Gateway 2.5 with Couchbase Server version 4.0?
On Q1) You may be able to, but this is not recommended or supported. Please see https://docs.couchbase.com/server/6.0/install/install-platforms.html
On Q2) Please check https://docs.couchbase.com/sync-gateway/2.5/compatibility-matrix.html