Can we request support for the management/upstream cluster and certain downstream clusters and not others?

No.  Mixing the management of "supported'" and "unsupported" clusters from a single Rancher server instance in this manner is not a supported configuration, so we would suggest that you run a separate Rancher server instance to manage clusters you do not want covered by Rancher Support. The other option you have here, would be to take advantage of our blended Platinum and Standard plan, to cover less critical clusters with the Standard plan, and you can reach out to your Account Executive, if you wish to look at the options with this.