Windows 7 XP Mode: is it OK to change the hostname and join to a AD

"It's ok to change the name.

Note that you'll need to change the saved logon credentials if you want to login as another user (like a domain user). This is done in the Virtual PC settings dialog for the XP Mode machine.

I'd definitely recommend joining the XP Mode VM to the domain for managing it, making it compliant with your systems and whatnot. Most things managed on the host should be equally managed on the XP guest like:

Configuration policies
Anti-virus software and definition updates
Firewall rules
Patch management
...and so forth

Some issues with networking could occur depending on the host configuration, requiring some further changes of the VM network settings to get it to successfully talk to domain controllers."