If you are looking at vmotion of VM between 6.0 and 5.5 hosts, then that is a supported use-case.
But make sure that the VM's running on 6.0 is having hardware 10 ( dont upgrade the hardware version to 11 which is the compatible version for 6.0)
Also, dont enable the new feature of 6.0 esxi on VM's.
You can upgrade vmware-tools to 6.0 version because tools are completely inter-operable across various version
BTW, when you are upgrading a vsphere 5.5 cluster to 6.0 , during upgrade process, some of the vm's will be in 5.5 and some in 6.0 and you will migrate them across different version when you put them in maintenance mode. So this is completely supported configuration.
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Yes, Mixed ESXi versions in a cluster works well. The only caution is no VM should have Hardware ver 10. So VMs can vMotion between 5.5 and 6.0.
Obviously ver10 VMs can't move on ESXi5.5. I did it , when upgrading my environment from 5.5 to 6.0. Some of the hosts were on 5.5 and rest were on 6.0.
Let me know if you need more details on it.
Thanks to both of you for the confirmation. I'll be upgrading the one host this weekend, then early 2016 3 new servers and finish the upgrade.