There really isn't a "right length" for a portrait lens -- there's a "right distance" for a portrait shot. The "right lens", then, is the one that lets you frame the picture the way you want when shooting from the right distance.

A 70-200mm lens is a great fashion/beauty lens on a full-frame camera (like the 5D); you can stand at a distance that's very flattering to the model and crop in tight on just the face. On a 1.6x crop factor camera (like the 7D), you've either got to be quite a distance away, or crop in very, very tightly.

The 50mm and 85mm lenses fall well within the bounds of traditional portrait-length lenses on a crop-sensor camera. The 50mm fills about the same role as the 85mm lens would in the traditional full-frame world; the 85mm hits about the same spot as the traditional 135mm lens. I can't say a lot about the quality of the bokeh of either of those lenses (I haven't been a Canon user since the manual focus F1n went out of production, so you'll have to rely on online reviews like those at photozone.de), but I can tell you that there will be sufficient quantity of bokeh with either lens. Which one you'd choose depends on the kinds of pictures you want to take. The 50mm is good for half-figure or better; the 85mm is better suited for head-and-shoulder, but it's also great for full-figure shots if you have the room. And do yourself a favour by taking a look at the 50mm f/1.4 and 85mm f/1.8 lenses, since they offer a significantly reduced depth of field for a lot less money than the f/1.2 lenses, and might be all you relly need.

For a very tight head crop, you might want to go a little longer than either lens -- and you might find that the 135mm f/2.8SF (soft focus) lens, which is a sharp lens when the SF is dialed to 0, is a versatile addition to your kit. When soft focus is right, it's right, and the effect from a proper soft-focus lens is better than what you can do in post.