Hello Dipak,
you can use different group numbers or you can reuse the same group numbers because multilayer switches allow for this.
On routers you need to use different group number in different vlan subinterface.
>> how hsrp get heartbit from core1 to core2?
you need a L2 trunk allowing all vlans of interest between the two core switches or the HSRP hellos will go via the access layer switches.
if you have L2 access layer switches a L2 trunk made of an etherchannel between the two cores can be a a good choice.
Hope to help
Giuseppe
