I built the Micro Engineering 210' tall viaduct kit into a curve. Going as small as you want with your curves may be pretty tight - I believe mine was ~30" radius.
I built a small wooden template of the track arrangement as it would go across the bridge, then I made the staight sections of the viaduct, laid them on the template at angles that would keep both sections "on" the template and cut a little off of the mating ends of the inside corners of the bridge sections to get them to "round" the curve. Remember - a curved bridge is a series of straight bridges connected together to follow a smoothly curved track.
With the small radii you're looking for, I would expect you need to widen the gap between the tracks as well to avoid trains bumping each other in the curve as they pass on the bridge.