If your AU is fandom-specific, it must a) include the fandom name in some form at the start, and b) only be requested/offered for that fandom (or closely related ones, e.g. Game of Thrones tags may be used for A Song of Ice and Fire).
Basically: yes, what you're suggesting re using tags from closely related fandoms is fine, if they fit.
no subject
From the rules post:
If your AU is fandom-specific, it must a) include the fandom name in some form at the start, and b) only be requested/offered for that fandom (or closely related ones, e.g. Game of Thrones tags may be used for A Song of Ice and Fire).
Basically: yes, what you're suggesting re using tags from closely related fandoms is fine, if they fit.