You can do this, but you can't (yet) do get_resource on your resource of type SecurityGroup.yaml and have it magically get the id of the resource inside.
In SecurityGroup.yaml you need to create an ouput that has the value { get_resource: server_security_group }, and then in your Launcher file use {get_attr: [SecurityGroup, <name of the output>]} to get the uuid of the security group to pass to other resources.