Dave, to help with question no.1, based on a 1965 Reading Co. circular (#856) there were 12 of these particular gondolas in container service. They were 20728,21544,21592,23015,23016,23017,23018,23019,
23020,23030,26553,and 26671. Somewhere in the "Modeler" forum is a few good pictures of these cars from Rich Yoder Models. If my memory serves there is a photo with the 10 drop bottom containers they carried. As for HO scale Westerfield Models makes a Reading set which fit these cars nicely. Now as to the accuracy of the Walthers Model in question I am uncertain. Tom mentioned the EJ Lavino iron works, in '65 they were assigned 34239 GHn gondola for container use.

Question no.3. I have used Scalecoat II on brass with no issue. My chief complaint with the this paint is the finish. I ended up thinning the paint with Lacquer instead with better results. Then of course baking the brass in the oven goes a long way. As far as the vinegar I heard it works to etch the metal, I have always blasted mine, then primed, then finished with desired color.