I replaced one nylon string (the G string) with a steel string. All the other strings are nylon. Is this okay?
You'll probably get away with it on a temporary basis, but the tension isn't good for that guitar. Certainly, on a classical guitar, I wouldn't be putting all 6 on. One won't be too detrimental, but also the feel will be different - tighter, thinner - and certainly the sound will be different, so it's inappropriate for many reasons.
Steel strings are available in a wide range of gauges, which will need different levels of tension to play at any given pitch. For the most part, one may reduce the tension required to play a certain pitch by using a narrower string, but as metal strings get too loose, pitch stability suffers severely--much more so than with nylon strings.
The tension on a 13-gauge steel string (the "e'" from an heavy-gauge set or a "b" from a lighter set) would be about 9.7lbs. That's rather light as steel strings go, but probably not unplayable, and it should be safe on a nylon-string guitar (many nylon sets average slightly over 10lbs). I would not expect such a string to sound very good or offer a good playing experience when used in that fashion, but the tension should be low enough to avoid damaging anything. Using a 14 gauge string would increase tension to about 11.2lbs; that would probably improve playability, and would probably be safe, but would leave less safety margin against accidental over-tightening.
Hey, it's your instrument; the guitar police won't break down your door for putting that string on your instrument. And it isn't like diesel in a gas engine; the damage won't be immediate and cause complete failure. The instrument will be playable until it won't.
But that's not what it's built for, and strings are cheap.