You can technically run mixed version clusters, but you do need to keep in the back of your head that everything in vCenter will need to remain at the lowest common version or else you will limit your guest placements. Example - if you upgrade VM hardware or VMTools past 6.5-compatible versions, then those VMs will only be able to run on your new hosts, thus possibly limiting availability. New features, datastores and VDS need to be taken into consideration as well. Example - 7.0U2 provides the ability to have a native key provider so that you can make use of encryption-based features. If you implement this and encrypt data stores or VMs then you'll run into some problems with your 6.5 hosts. I personally recommend to run mixed mode temporarily. Perhaps you can split the workload, i.e., non-prod on the 6.5 and prod on the 7 or something along those lines.
