Ah crap, my post disappeared.
Schools look neater if you only have one school of one species and color per level, but as long as you have the recommended six plus per school, it is entirely up to you and your aesthetics.
As far as the ghost catfish (more commonly called the glass catfish, species name Kryptopterus vitreolus), seriouslyfish.com recommends a tank base of at least 90 by 30 cm. This is also a schooling species so, again, you'll need more than six. They are also incredibly shy, so the more the merrier. However, you should keep in mind that these guys get up to 65 mm, so the more of them you have the more tank you'll need.
Having said that, two schools of corydoras need about forty gallons for a long tank. Then, if you add in the larger school of the glass cats, you're looking at a very large tank.
The best question would be how big of a tank you are willing to buy and how much money you are willing to spend.