1. Do I have to upgrade the vCenter Server first to version 6 before upgrading the hosts to the new ESXi version?
Yes
2. Can vcenter Server manage ESXi hosts with versions higher than the vCenter version is using?
No
SRM:
1. Do I need to have the licenses for SRM for PROD and DR sites in both failback and reprotect scenarios?
2. Is SRM license will be the same for both PROD and DR sites?
Please refer this FAQs document, page 3 of this has all the answers regarding licensing for SRM
3. Do I need to create placement datastore in both replication using array and vsphere replication scenarios?
4. If yes, do both sites need to have at least one placement datastore in failback and reprotect scenarios?
5. What is the use of placement datastore? Do it have to be large?
Please refer following thread, that's where your questions about placeholder datastore are answered
