Perhaps there is some confusion here, as I understand it you have a DPM Server and a Hyper-V host with a number of guests. You have deployed an agent to the Hyper-V host and you are configuring protection of the guests. You have added a new NIC to the Hyper-V host for all DPM traffic to flow to and from the DPM server. Would this be correct?
As far as protecting guests, DPM utilises the VSS service to protect the VHD files. If you have the Hyper-V integration tools installed on the guest, this is done seamlessly with no downtime. If the integration tools are not installed, the guest will be paused while a snapshot is taken.