Yes, you should clip one wing on each duck. This is for their own safety. I had mine fly to the neighbor's yard and not be able to get back over the fence. One ended up three houses down and I wouldn't have got her back except they knew I had ducks and called me. There are good videos on Youtube...
How they look depends on the runner's color. My mallard girls crossed with fawn-and-white runner drake tend to come out buff or light brown. Some have a white ring around their neck.
I suppose the runners can fly if they practice. They are not heavy birds. Mine has no reason to fly. His buds are all ground-bound. They are either non-flying ducks or they have their wings clipped.
I have hatched quite a few of the reverse cross, a fawn and white runner drake with a mallard. The resulting ducks are either all buff (strangely enough) or medium brown. In my avatar pic, the drake in the center and the girl on the far left are runner x mallard. This is a wonderful cross, IMO...