I don't even think the "guaranteed availability" rule itself is what's in question. If a small station knows a PC is coming to get a particular type, it should definitely make sure that type (or something better) is made available. Yes, they should absolutely be on the phone, driving a suitable car directly from the airport to the train station for you.

If Hertz.com was showing sold out at the station completely and you invoked the PC guarantee to get a car, I'm not sure how much leeway the rules specify as far as specific car or transmission type. When I was PC, I only invoked it once - in the U.S. - and I was pretty clear that I'd accept any car smaller than a van. In Europe, I'd just assume I'd probably be getting a small car with a stick since they are so common in rental fleets.