You would have to change only userdata that is passed to instances.
Zend Server instances expect it to be valid JSON object. If you open CF template you will see that JSON is constructed there dynamically to indicate details about RDS instance.
Instead of constructing JSON dynamically you could make it static with details of RDS you created manually.
To have multiple clusters use same RDS, you will have to use separate DB for each cluster. DB name can be passed using ZEND_CLUSTER_DB_NAME parameter of JSON object passed in userdata.
Cloud Integration Engineer
Zend Technologies, Ltd.