Can i have a vCenter Server Standard on one site and a vCenter Server Essentials on the other site for vSphere Replication ?
When you buy essentials you can run.
1 vCenter Server.
3 ESXi Host servers with upto 2x CPU Sockets and 64GB RAM in each server. (so 6x CPu Sockets and 192GB of RAM over the 3 servers)
If you don't need the features of the standard license convert it to an essentials license and connect it to the same vCenter Server.
From a replicaiton perpective. It would depend on the replication technology / product you were using and the verions of ESXi you are running on the Hosts.
for vSphere Replication feature you need at least Essentials Plus license...so if you upgrade your license from Essentials to Essentials+ you will be able to leverage vSphere Replication potential.
Reffer to this document:
You can also replicate VMs within single vCenter instance i.e. (meaning single site with Ess+ license) for more info see:
Replicating a Virtual Machine in a Single vCenter Server Instance
For other scenarios have a look at this white paper:
http://www.vmware.com/files/pdf/vsphere/VMware-vSphere-Replication-Overview.pdf