Most rental companies have cars from several car companies, but they tend to be heavy in one company's cars. From what I remember:

Hertz: Ford (though more non-Fords than in the past)
Avis: GM
National: GM
Budget: Ford
Dollar: Chrysler
Thrifty: Chrysler
Enterprise: varies by location

But sometimes, rental car classifications are odd. Sometimes, for example, an "intermediate" car is a model with less interior room than a "compact" car at the same rental company. You may want to look at the rental car company web sites to get an idea of the more common cars they have for each size.