Should I put everything in one Service Group, or should I have more than one Service Group?

Usually people try to separate different applications as much as possible. Service Groups serve as logical divisions for your applications. You don't want a failure of one application to cause a failover of all your applications if its unnecessary. If all your applications are using the same Group, then a failure in that Group can cause all your applications to fail. The goal of high availability is to try to minimize single points of failure. That is why separate applications in a cluster usually means separate Service Groups are recommended.