I believe House Frey is a MINOR House. They are in lieged with House Tully. As I understand it, there are more than 7 Houses because kapag yumaman ka sa Westeros puwede kang maging "House" but you have to pledge allegience to one of the 7 major Houses na mas mayaman and mas elder sa iyo. Kapag nasa North ka, you pledge to House Stark, kapag nasa Riverlands ka, you pledge to House Tully.