As far as I know, there is not an install guide specific to Xenapp, which appears to be a Citrix virtualization software.

The installation guides can be access from the below link and you will need to specify the application server and database in use:
https://community.pega.com/knowledgebase/products/platform

Still don't see an install guide specific to Xenapp 7.15, will there be one or is the one for 6.5 valid?

Which Pega product and version are you wanting to install on Citrix XenApp?