1. Yes, that's it exactly. We collect topology, performance and event data through the vCenter servers, so you only need to configure the 7 vCenter connections.
2. Yes, so long as there are sufficient licenses, the default is to extend monitoring to additional hosts automatically.
In the nworks Enterprise Manager UI, you can override this default behavior. For example, if you have a vCenter that is managing mostly non-production servers, you can configure the connection to not automatically monitor hosts by default, and then extend monitoring to only the hosts you want through the UI. In addition, once hosts are discovered, you can uncheck them to stop monitoring, which willfree up licenses to use with your other hosts. The default, however, is to monitor any host attached to a VC automatically.
3. You should see clusters reflected in messages based on vCenter events and measurement thresholds, the integration with the service map (which visibly reflects clusters in the topology) and the reports. We do have reports that deal specifically with clusters, but it's not as central a focus as reporting on virtual machines and hosts. Are you working with one of our solutions architects? They should be able to provide additional information about the reports, as well as take feedback if you have requests or requirements around the reports, or any other aspect of the SPI.