Although you can remove both from the server, it is not recommended to remove the DNS server part, because this handles the translation from server name to IP address.
Then of course, there is always the ability to have 2 DHCP servers running concurrently. Configure them the same, but make sure their scopes do not overlap. This way, you always have a fall back. But this is not required. You could entirely use the DHCP server from the router, which, in this case, is preferable.
What you want to do is as follows:
Make sure your server has a static IP address outside of the DHCP scope.
On your router configure the DHCP server to hand out IP's as follows:
- IP range with subnet mask that is free
- DNS Server1: server 1, your server's IP address
- DNS Server2: Either the one of your ISP, or 8.8.8.8 (google's DNS server)
Do note that there is a risk with this setup. If your server goes down, DNS will not be translated anymore. The client will still be using this DNS server until they get a new leash from the DHCP server, so the client needs to reboot until the server problems are solved.
This is not best practice, but for what you want it should do.
Best Answer
Active Directory doesn't require DHCP, but it does require DNS. You'd need to set up some sort of DNS for it to work (Microsoft, BIND, etc). Most people use their Active Directory server as their DHCP and DNS server as well, though, as the integration of the three makes things easier.
I'd set up your server as AD/DHCP/DNS, disable the DHCP and DNS forwarding on pfSense, and tell your MS DNS server to take care of the DNS forwarding.