Active Directory Activations

Can we run both KMS and AD activation together?
Yes, you can, in fact you must because AD activation only works on win 8 and above and windows 2012 and above server OS

Do we have to have KMS in place until we retire the windows 7?
Yes, that is the only because ad activation cannot be used for win 7 / 2008 R2

Which activation key we have to use it  for AD activation configuration, is it the old KMS?
KMS key should be used based on OS you will be activating
Use 2016 server KMS key to activate all server OS from 2008 to 2016 and win7 to win 10
https://docs.microsoft.com/en-us/windows-server/get-started/server-2016-activation
https://docs.microsoft.com/en-us/windows-server/get-started/kmsclientkeys

I did not see / here any issues for AD activation till date

I read the link below. So I have to put multiple KMS keys(Windows 2016 and Windows 10) in the KMS key box, am i correct?
https://docs.microsoft.com/en-us/windows/deployment/volume-activation/activate-using-active-directory-based-activation-client

That where  I am confused, if we use Windows 2016 Key then it should cover all below version. Why do we have to have old KMS then?

Because AD activation service cannot activate win 7 & 2008R2 OS
For those you need kms server

U need to retain current kms server setup to activate win 7