[Should I use nlite or XPize itself to integrate?]

I havn't tested it but it may work. After normally integrating xpize .EXE file as hotfix, use the addon maker on xpize with /S /nofiles switch and again integrate the generated .cab file into nlite hotfix page.

Edit:

Tested. It works . You can ofcourse make an addon with /S switch alone and integrate just that instead of two files as in above method, but that wont show the royale-theme-colours while installing windows.