my only road block is that if i upgrade ESXi 5.5 to 6.5, then does it make any problem by rest ESXi host running ESXi 5.5 in same cluster?
The best practices is to have all hosts within the cluster running the same version and build number, but no problem to have different ESXi versions during an upgrade.
does it get any problem with VDs version 5.5 ?
Your ESXi 6.5 will support the vDS 5.5, since the vDS 5.5 is supported by ESXi 5.5 and later. And after you upgrade all the hosts connected to the vDS, you can upgrade the vDS version.
