XenApp 6.5 upgrde in Windows 2008 DC vsphere data center. APP requirement onf windows 10 ws

Hello experts, it's been  a while since posted.

I have just started planning out a Xenapp upgrade project because there is a mission critical business app alled "BLUE" our users access that has to be upgraded.  Users are on wndows 7 majority in a domain env that runs in VM.  Our datacenter is running 6.5 vcenter , 2008 DC operating systems for servers we deploy, including our two Xenapp servers.  The business app BLUE requires all workstations be either win8x64 or win10x64. So Xenapp we decided to optimize and use to publish the app.

Is what I'm discovering true,  you have to be on server 2012 R2 for win10 and can't do an in place upgrade of xenapp?  
I'm not that surprised at the latter no in place upgrade path from 08 to 2012, if so... but obviously adds to the cost, labor and time...

Also, if so we are not quite ready to invest in  Win2012DataCtr os licensing, can we mix 08 DC with say a few guest servers running 2012 R2 Standard lic and not 2012 Datacenter in our datacenter?

Thank you.