I could be wrong, but to my understanding you don't need to create new REST api services to scale out. It should be sufficient to add a new host to the cluster, add it to the same group on which you already created a REST api, and you can access the REST api on that new host.
It won't have its necessary modules on its local disk, so needs to access the cluster to retrieve the modules from the original host. It is likely that most will be cached though, so that would mean an initial delay only.
The only trouble would be with running custom files from file-system. Those would need to be installed on each host individually. It would also implicate that if you upload custom extensions to the REST api on one of the hosts, it becomes available on all others as well. They literally share resources. It makes scaling easy, but might not be what you functionally need. In that case, Erik's suggestion to use Packaging could be usefull.
Only if you are talking about duplicating REST api's between clusters (or groups within a cluster), you really need to recreate the REST api. There is a section in the REST api guide about this topic:
http://docs.marklogic.com/guide/rest-dev/migration#chapter
It might also be worth reading a bit about working with MarkLogic clusters, particularly the distinction between so-called Evaluator-nodes and Data-nodes. You can find a introduction on that topic in the cluster guide:
http://docs.marklogic.com/guide/cluster/distributed#chapter
HTH!