Yes. At the top level call it Car Rentals and Programs. That forum would have all the general (as distinct from the pre-1993 company called General Rent A Car!) discussion.
Have subforums of Car Rentals and Programs for the larger companies and their programs. Larger by importance, not by ownership, and specifically excluding Enterprise. Consider having a subforum for Opaque Rentals (Hotwire, Priceline).
Excuse me for asking a perhaps naive question, but why not Enterprise? I've never used them but AFAIK they're a major rental car company.