Elasticsearch Multiple Cluster Search
No, I think a search via the api or your client of choice (Java/Python/etc) is going to be against a single cluster.
Your client could make multiple requests one to each cluster, perhaps if you organized your clusters by years?
In theory a cluster could just grow forever, although at some point I would think the overhead of scattering and gathering a query to N nodes (where N is very very very large) would cause problems.