nylon
nylon mostly
The term acoustic, albeit not neccessarily accurate, usually implies a guitar with steel strings, whereas classical implies the use of nylon strings. For playing almost all types of modern music, a guitar with steel strings is preferred.
String cheese, Sock string
Nylon strings.
Yes, i do regularly. You will not get the same resonance as a steel string but you can and well.
If by "classic acoustic guitar" you mean the small bodied, slotted-headstock type of guitar commonly called simply a "classical" guitar, the answer is an emphatic NO. Any type of steel strings on a true classical guitar will ruin it. Classical guitars must be strung with nylon or gut strings only.
it has 6 or 12
Most classical guitar strings are made from a sort of nylon material. Some expensive types of 6 or 12 string guitars used for Mexican music use steel strings.
A classical guitar IS an acoustic guitar. Now, if you are asking, "How can I make a classical guitar sound like a steel-string flattop guitar", the answer is: You can't. The only way you could approach that is to put steel strings on the classical guitar, and if you do that, you WILL, not "maybe", FOR SURE, destroy the guitar. In fact, the bridge may pop completely off the guitar while you are tuning up for the first time. If you want steel-string sound, get a steel-string acoustic. Please don't ruin a perfectly good classical trying to get that sound. This is 45 years of being a guitarist talking, here.
Nylon strings are lower tension in comparison to steel strings and have a warmer tone than steel string, thus making steel strings higher tension than nylon with a brighter tone than nylon strings.
no, a classical guitar doesn't always have a truss rod. The added tension could warp the neck making it unplayable over time. In my experience classical guitars also have softer frets than a steel string guitar. Steel strings would eat away at a classical guitars frets in no time. I've personally witnessed classical guitars that were ruined because someone wanted to use steel strings on them.