Yes, I agree with Nitesh.
You can add mixed version hosts into cluster but it is recommended to keep host inside the cluster on same version.
Some implication if you have mixed host cluster.
1. Say you have 5.5 host, you can easily create VM with VMDK size more than 2TB, but when DRS thinks it needs to be migrated to other host say 5.1, VM can not be migrated as host 5.1 supports VMDK size upto 2TB. Hence it is going to reduce the DRS opportunities to balance the memory & cpu load.
2. 5.5 host can have VM with hardware version 10, same can not be managed on host with 5.1 version.
3. In 5.5, HA can be made aware off VM-VM anti-affinity rule but on host 5.1 HA will not honor VM-VM anti-affinity rule. Same is the case with DRS with flash resources.
4. Also, every release there will some improvements on various features.