Question: Can a single DM orchestrator support a mix of on Prem and Cloud environments?
Answer: A Deployment Manager instance can support any number of candidates which are running 8.1+. Candidates environments are not required to be on the same version. If you can manage any network connectivity issues allowing both the on prem and cloud candidates to communicate with orchestrator then that would be support. This may require moving the orchestrator into a DMZ or enabling a VPN tunnel between your networks. This would not be supported on PegaCloud.
The alternative is to have two different Deployment Manager instances.