According to Astrology.com, "Capricorn, the tenth Sign of the Zodiac, is all about hard work. Those born under this Sign are more than happy to put in a full day at the office, realizing that it will likely take a lot of those days to get to the top. That's no problem, since Capricorns are both ambitious and determined: they will get there. Life is one big project for these folks, and they adapt to this by adopting a businesslike approach to most everything they do. Capricorns are practical as well, taking things one step at a time and being as realistic and pragmatic as possible. The Capricorn-born are extremely dedicated to their goals, almost to the point of stubbornness. Boy, those victories sure smell sweet, though, and that thought alone will keep Capricorns going.
The Goat symbolizes Capricorns, and an apt mascot it is. Goats love to climb to the top of the mountain, where the air is clear and fresh. In much the same way, Capricorns want to get to the top of their chosen field so that they can reap the benefits of success; namely fame, prestige and money. Getting to the top isn't always a walk in the park, however, so it's likely that Goats will ruffle a few feathers along the way. These folks can indeed be domineering, even egotistical, on their route to the top. They'll tell you it's part of being a leader with bright new ideas (in keeping with the Cardinal Quality assigned to this Sign)."
One of the reasons that Kyo despises Yuki is that no matter how hard Kyo works to get better at any given activity, Yuki is automatically better than he is without having to put any effort into it at all. Kyo is definitely a hard worker: he puts a lot of time and effort into his studies and trains regularly to get better at martial arts. He definitely works hard to get to the top of that proverbial mountain, but he seeks freedom rather than material reward. His continuous challenges to Yuki are a fine example of the ambition and determination that Capricorns tend to possess.
Is Kyo practical? Despite his excitibility, I would say so. He often gets exasperated at some of the silly ideas that come out of the mouth of his friends. He's often the first one to reject outlandish suggestions and he has little interest investing time and energy into things that do not interest him. However, I wouldn't say that he has a buisnesslike approach to life as he's rather passionate and emotional.
Does Capricorn fit Kyo's personality? I'd say so! Astrological zodiac signs and horoscopes are often worded so generally that anyone can fit the personalities they describe but I'd say it's more spot on for Kyo than not.