Most everything except the largest engines and long cars can run on 9 3/4. Long cars will have objectionable overhang which can interfere with scenery and tunnel portals. Even the latest articulated steam are supposed to run on 9 3/4 radius, although they don't really look that good doing it. Most diesels will be fine with 9 3/4. Long wheelbase steam and long wheelbase diesels you will want to stay away from but if you are doing something small, a little road switcher is really all you need to run anyhow. It's not like you will be pulling 25 car trains. Just figure on a small layout, no long passenger, auto racks, or intermodal flats etc. and you will be OK.