Hi there Haider,
Since I got the same questions in our support ticket # 00793811, I'm pasting my answer here as well:
Q1. It is not recommended to make your Core a PXE Representative:
Q2. You can as the PXE representative is able to act as a server for multiple client at a time.
Q3. Our recommendation is to use a PXE Representative for each subnet where you have target devices to provision. This is the simplest scenario. However, it is possible to use single PXE Rep for multiple subnets with a proper configuration of DHCP Relay Agents and DHCP option, as explained in this community article:
How to use a PXE Representative for multiple subnets
Hope this helps
Q3 You need to add ip helper ( if using cisco router) on the target Vlan. of the PXE server, Proberly as you do with the DHCP server.
Vlan 430 "Client Vlan"
ip-helper [ipadress of PXE server]