I agree that a separate AD only in a smaller school is not feasible financially for the school. However, a combination such as AD/Asst. Supt could be a possibility. Obviously, if your school and community are mainly concerned with the success of the football program, the AD will always be the HFC. Nothing wrong with this but this often cuts down on the quality of candidates for the other head coaching positions if these sports don't get (in their eyes) a fair chance at success.
Particularly in East Texas, I think most communities are fine with the AD/HFC combination and many times this works for all. I think more than anything else it comes down to the character and leadership of the AD and not the sport this person is the head coach of.