The oldest guitar tutor books don’t say much about players’ nails but from Sor and Aguado we can find some hints. Sor has said that he cut his thumbnail in the fashion of his good friend Aguado. That means that he probably used nails on other right hand fingers. From Tarrega we know that for the last decade of his life he played without nails. There were reasons for that but it means again that before that he probably used nails.
If we see nowadays lute players as well as the historical guitar players, we notice that they don’t use nails. It is said to have been a common practise on those gut-strung instruments. However, there are exeptions. A lutenist called Piccinini probably used his nails for chamber music. On a modern classical guitar most of the players use nails. Mostly children and other beginners play with fingertips only. It also goes for those rock-guitarists who once in a while try to play a few tunes on the classical instrument.
These are the facts. But why do we need the nails for? For the clarity of sound and for the loudness. Those old instruments that I mentioned had as low string gauge as 2 kg per string. It is possible to get it sound without nails. The modern high tension nylon strings need much more than that to project the sound over a concert hall.
The length and shape of nails that players use is very different. There are players who use long nails so only nail is involved in sound production. The main principle is that the plucking action has to start with both the flesh and the nail touching the string. Longer nails give some people a little bit better accuracy. So they say at least. If the player is used to a certain lenght of the nails then half a millimeter of diffence causes most likely many problems.
See Scott Tennants Pumping Nylon for his ideas about guitarist’s fingernails. He has pointed out some general rules of the shapes and angles for polishing. Yes, you have to polish them. All the time. By the way, I haven’t broken a nail for at least 15 years. I guess I’m just lucky. The only thing I cannot do is to play basketball. Volleyball is OK, though 🙂