So perhaps that will complicate things
Yes it will. dhcp works off from broadcast messages, so it communicates by sending out broadcast messages on the same vlan/network. Having 2 dhcp servers on the same network segment will cause you grief.
Fog can integrate into an environment where there is an existing dhcp server no problem. FOG’s internal dhcp server would be typically used if you were to setup an isolated imaging network where there was no device except the pxe booting computer and the fog server.
So lets start out with this, what device is your preexisting dhcp server?