Something to keep in the back of your mind is that you may run into a circular dependency scenario, where multiple resources depend on each other for creation. A common example is where two security groups reference each other in order to allow access between themselves.

A workaround for this particular scenario is to use the AWS::EC2::SecurityGroupEgress and AWS::EC2::SecurityGroupIngress types instead of the ingress and egress rule types for AWS::EC2::SecurityGroup.