The DHCP server needs to be on the same subnet as the RDM server, but doesn't have to be on the same server as RDM. In fact, from testing we have been doing today, it performs a lot better if the 2 functions are kept apart.
Assuming your client systems are all in the same subnet as the DHCP and RDM servers, all should run fine.
If there are clients in another subnet, and they can get a DHCP allocated address already, then you must have dhcp/bootp forwarding active on your routers. In our case, Cisco routers, using the 'ip-helper' command to define the IP address of the DHCP server.
To get RDM working for those clients, you have to also use 'ip-helper' or equivalent, to define the IP address of the RDM server.
i.e. you have 2 helper addresses - 1 for DHCP, the other for RDM.
PS - if you are keeping DHCP and RDM apart, make sure the DHCP scopes do not have option 60 configured. You only need that if DHCP and RDM are on the same box.
Hope this helps.
Rob