I have a network topology of 6 edge cisco switches 3560 aggregated on cisco switch 3750 and a dhcp separate server feed all the network clients PCs with their IPs.

I want to make a dhcp on one of the edge switch to feed the connected PCs on this switch only and leave the main dhcp server as it is feeding rest of network.

How to be done without conflict between the 2 dhcp servers

You can have multiple DHCP servers on the same network as long as they are not handing out the same Scopes (address ranges). They will answer on a first come, first serve basis.

To isolate the servers from each other, you can place them in separate VLAN's. Is this possibly for a VoIP implementation? There are also some custom entries you can make in your Windows based DHCP server that will refer the IP phones on to the DHCP server for the phone system.

You can assign different subnets to each VLAN and then use VLAN routing as well. This way, your DHCP servers will be handing out completely unique data (not just different IP addresses) for the VLAN that they reside in.

For QoS purposes, you would also want to use separate VLAN's for voice and data.