Yes and yes, either/or. The #58s are scale-sized, not oversized like the #5s. The 's' doesn't mean anything, it's just plural (as in, fives vs 5s).
As far as the couplers mounted to the 'bogies', the normal term is 'truck mounted couplers'. Sometimes, you might hear it called "Talgo mounted couplers'". Pretty much the same thing.
You can convert truck mounted to body mounted by cutting off the protrusion from the truck, and mounting a box onto the chassis, like your other cars. Many of us do that, on some passenger cars who use truck mounted couplers because those handle small-radius track better (like 18-22" radius). But, when you are backing up, they have a tendency to derail, because the pushing force is on the trucks, and that might force them upwards, off the rails.
Price-wise, if you go to the Walthers site, their prices for these items pretty much mirrors retail (no discount). Certainly, stores will sell at that price, or less (since Walthers is also a distributor).
Kennedy