Hi Abhayms,
1) WLK 1.6 is still valid for certifying drivers upto Windows server 2008R2\ Win7 OS. HCK 2.1 supports from Windows Server 2008R2/Win 7 till latest OSes. So, Win 7 is mutual between both versions of kit. You can use either of kit for Win 7.
2) If you are going to use HCK 2.1, you have to complete all test cases or if you are submitting partially submission, then you need Manual Errata for remaining test cases which are not applicable for your device , for this you have to contact SYSDEV.
3) you can't install both kits on same server. you have to use one kit only on a server.
Thanks,
Mudit