Myth and truth
You're here because you want a brighter sound on your classical guitar like on the acoustic and you can't stand that muddy nylon sound anymore. But isn't it dangerous to put metal strings on a classical guitar? Wouldn't that screw up the guitar in a few minutes or so?
Yes, we all know the story, metal strings on a classical guitar breaks the instrument in two, rips the bridge off the top or bends the neck like a bow and so on, because classical guitars don't have truss rods inside the neck. Yea, yea, yea...
First of all, you miss the point, or the scientific factor: the tension of a string, regardless of the material. That's the most important thing you should take into account when purchasing any kind of strings.
The story about the screwed up guitar after getting metal strings could be true, but only if those strings are high tension bronze "wires" for acoustic guitars (I hate those so called strings). That's the only case.
So, yes, you can put metal strings on a classical guitar, ie. electric guitar strings. Some nylon strings have even a higher tension than electric guitar strings and might damage (bow) the guitar in time more than those "scary" steel strings.
Facts:
Nylon strings have an average tension of about 15 lbs (~7 kg). Guess what? The average electric guitar strings have the same tension! (You gotta be kidding me...) And, of course, like I said, acoustic guitar strings (the really scary ones) have double the tension of those (about 30 lbs or ~14 kg) and might actually crack your classical guitar in two (it could be a matter of minutes or hours before you hear the "explosion").
Saying that, have fun buying, mounting and playing electric guitar strings on your classical guitar and relax, because they won't do any harm to your instrument.
But be careful not to mix up electric guitar strings with acoustic guitar strings. The strings for electric guitars are nickel wounded steel strings and for acoustic guitars they are bronze / phosphor strings. It shouldn't be hard to chose the right ones. Just read the information on the package before you buy them.
One more thing I should tell you about. Tension also means gauge. So, the average gauge of electric guitar strings is between about 10 and 40 (0,010 - 0,040 in). For instance, I'm using D'Addario EXL electric guitar strings of 9-42 gauge with an average tension of 13,5 lbs (~6 kg) on both my electric and my classical guitar now.
