A Boot Server includes:
- DHCP server component (DHCP Server or proxyDHCP Server)
- File Server component (TFTP/HTTP/NFS/CIFS/etc).
The Boot Server DHCP server component can provide
- IP address + booting information (DHCP Server)
- Only booting information (proxyDHCP Server)
If you already have a DHCP server in your network when adding a Boot Server you have to either add the booting parameters to your existent DHCP Server or set the DHCP componet of your boot Server in proxyDHCP mode
In any case having more than a DHCP server and/or proxyDHCP server in the same network segment is always BAD; because you are exposed to DHCP collissions, races, etc, etc.
The best idea is having just only one Boot Server with DHCP/proxyDHCP services enabled and configure its menu options to "chainload" to any additional Boot Server/s which must have their DHCP components turned off.
For your specific problem there are third party PXE server solution accelerators that easily integrate under the same boot menu Windows and non-Windows asset deployments.