I don't think that you can change DHCP server for a bridged network because a network broadcast is used to get to obtain the address of the DHCP server.

For a custom NAT network in Fusion Pro, there is an option not to provide DHCP services on the network. That should allow a custom DHCP server that's configured on a VM in that NAT network (will need a static host address, plus the DHCP server bits installed and configured) to be used for other VMs in that NAT network.

I just tried this in Fusion Pro and can confirm that no DHCP server is available to serve up an IP config in the custom NAT segment