Well, Terragen is a toolbox, there are no ready-made terrains either, so you'd have to experiment and use the fractals to get what you want indeed. In the cloud nodes you can set altitudes, thickness, hardness, coverage, mask locally, and in the power fractals or density fractal you can alter the noise in a lot of ways, combine noises, mask combined noises by altitude or 'strength' of the greys in the fractal, stretch them, warp them, etc...
Welcome to the forum, btw, and I hope you can find your way. There is a cloud library that may help you out to get an idea how to work on clouds.