is it possible to install VC appliance version 7 machine with my existing Vcenter 6.7.
not comfortable of upgrade but rather install a new VM instead of upgrade is it possible? if not, can you provide me upgrade path and documents.
I's not the web client's version that's important, but the vCenter Server version/build, as well as the ESXi hosts' versions that are managed by your vCenter Server.
Anyway, you can find details regarding supported upgrade paths, and supported ESXi versions at VMware Product Interoperability Matrices.