Hello Nick,
Yes, adding UCS based ESXi hosts to an existing vCenter installation is very much supported. I don't believe we have a best practice on this issue - we are flexible. Please note the actual support may be provided by VMware or Cisco depending upon contract.
Yes, both N1k & VMware vDS may exist in the same vCenter, datacenter, cluster, and even ESXi host. Only limitation is ability to provide enough vNICs (vmnic) to support external connectivity - not an issue with Cisco VIC (M81KR/VIC1280/etc).
Matthew