Flat top rail is essential. The big difference in wheels is that the 2-rail wheels have much smaller flanges. They do not like round top rail like the old tubular track. They run fine on old Gargraves. I had a layout years ago that was all gargraves, including switches and I ran 2-rail stuff without many problems. I do stick with 40ft cars for the most part.
As for the trucks, it is easy to go 2-rail to 3-rail. 2-rail trucks/wheelsets are insulated on one side. 3-rail are not. If you put a 3-rail axle on 2-rail track, you get a dead short.
You do need avoid kinks, your curves have to be smooth. Also you need to avoid big dips and bumps in the track. Shorter cars handle the dips and bumps better than longer ones.
Smaller cars obviously handle smaller radii better.
