Andrei, I had the misfortune to witness Baryshnikov's Siegfried, twice. He must have been more clever when he was young.
I think there are balletmasters today who don't know what employ is, and, even if they did know, would scorn it. Perhaps this is the American influence? Employ is not an American democratic concept.
Your putting it in prince, lover, fool terms makes good sense. Part of it is character, but part of it is also body type. (And for the Danes, a good bit of it is the nose. I'm not kidding. Turned up noses are demis, long, straight noses are nobles, tragedy.)
Michael, I think allegro/adagio makes a lot of sense too as a category. A languid dancer probably won't look her best in a role calling for quick footwork, and speed queens don't know how to take the time to unfurl their limbs in supported adagios. There are also other divisions, like "classical/romantic." I think a lot of this is lost. I'm comfortable only with 18th century and early 19th century because dear Noverre wrote it down, and historians quote dancers and balletmasters about how it evolved in the 50 years after Noverre's Letters. After that, I'm trying to glue feathers together to make a duck.
I also think there are lots of subtle distinctions within the genres that have gotten lost. Each of the fairies in Sleeping Beauty seems different, to me, and look best when danced by women who suit the role physically. I just don't know the names.
Victoria, I think Tudor was very careful about casting, too (except for his Leslie Browne period

) I don't know enough about Tudor to comment on how he used employ.
When the Stanislavsky Ballet came to DC, I thought they used employ better than most larger companies today. They knew who was the Prince and who the Jester, and those two dancers looked as different as night and day. But in the contemporary ballets (contemporary classical ballets) the same two men danced side by side and the distinctions were blurred. One was slightly taller; that's all. So a lot of it is how the body is used.
Alexandra