You use the <pre>service dhcp</pre> command to enable DHCP server. You have to use <pre>no service dhcp</pre> to disable DHCP server.
No you do not have to…but you have to have different scopes
If you’re going to use a server for DHCP, then you may want to disable it on the router. You will need to set up a DHCP-Relay to the IP of the server as well…