I checked the wiki on tostadas and their definition fits with mine except they claim that tostadas are usually topped by meat of some sort. In my vast experience eating Mexican food in restaurants this hasn't been the case.
For a start, the tostada is never the main feature of the meal. That is to say, I've never seen a tostada plate. It's always just served as one of the list of items included on a combination plate along with an enchilada and a taco, some guacamole, rice and beans. Maybe a tamale. I've had some variation of this meal in literally hundreds of restaurants. And the tostada is always the same. The flat round tortilla with a layer of refried beans topped with lettuce, tomatoes and cheese. No meat. If I'm wrong, I'm wrong, but so are hundreds of Mexican restaurants in Houston and many other places.
As for tacos, it's common to find refried beans in a breakfast taco, but we are talking about crispy tacos. And by the way, this is not entirely an American invention. The preformed and packaged taco shell is, but the idea of a deep friend crispy tortilla shell on a taco is as traditional as can be. And I've seen tacos filled with beans too, but it was for some vegetarians that special ordered it. I've been to many many
taquerias in my lifetime and they typically have a board listing what meats they use to fill the tacos. The list includes some offbeat choices like Lengua (tongue) (delicious btw), de cabeza (brain) (not what I was expecting, but not something I would ever want again) and tripas (tripe) (no thank you) along with the more usual choices of barbacoa, fajita, chicken, etc. In all my experience I've never seen frijoles on the list.
But again, we are talking about crispy tex-mex tacos and I've never seen one that was filled with beans and I have been served hundreds of them. They are almost always filled with seasoned ground beef. Sometime In the last two decades a chicken variety has appeared. Topped with lettuce, tomatoes and cheese.
So really, other than the bend in the tortilla the only difference is that a taco has meat and a tostata has beans. Again, if I'm wrong I'm wrong, but so are many many restaurants. I believe I've surveyed a large enough cross section to say this is a universal fact.