The widest range of tone color and dynamics is achieved with a combination of nail and flesh. By varying the angle of the fingers one can use either more or less nail. The optimum length is one that allows you to just see the nail over the tips of the finger when looking at the palm. The nail should follow the contour of the fingertip. It is also important to make sure the sides of the nails don't have any corners that will catch the string or catch other things and crack or break. By keeping the edge finely polished, you will not only help prevent accidental breakage, but will improve your tone as well. The edges should not be plainly clipped or left coarse, even to slightest degree. 600 grit paper or the finest polishing boards will do wonders. Treble life will be extended and you will avoid sounding like you are trying to navigate through barbed wire or have just dragged your nails along the concrete.
As for polishes and strengtheners, they are fine if they do not make the nail brittle. I recommend, and sometimes use, a product by OPI called Nail Envy. It is a nail strengthener / polish that also comes in a Matte Finish that is not shiny and is nearly undetectable. It will help keep your nails from breaking also. It is available in most nail salons, malls etc.. It is easy to remove with an opposing thumbnail, without the use of the acetone (acid) found in most polish removers. In my opinion, Ping Balls are just plain silly, but there are many great players with advanced degrees that use them. They do sound great, but super glue should only be used to repair nails, not to plaster things on to the nail bed. Some people have a really bad reaction to it. Really, Now? It contains cyanide, Poison!!! If one is not poisoned, then they risk developing incurable nail fungus. For repair, a little china silk (fabric), which has its own self adhesive, can be used over the top of the superglued and buffed repair. A very thin layer of the superglue is then applied over the "top" of the silk. A minimal amount will penetrate and provide additional bonding to the actual nail. The glue will make the fabric transparent and create a very strong overlay which can be buffed and polished as desired. When the nail grows out the overlay is easily removed, nail is rebbuffed, all without damage to the nail bed or the need for acetone.
It is true that a lot of teachers advise beginners to avoid the hassle of nails. This advice is best for younger students. Older students can often learn to use nails right off the bat. Fingerpicks and thumbpicks were not designed for nylon strings or classical players. Many techniques are just plain impossible with them. Speed is hampered, too. Steel string players should also beware. They may lead to serious tendenitus, such as what happened to Leo Kotke. They nearly ruined his career. He was very apologetic to audiences for the two years he spent recovering from their ill effects and making the transition to nail and flesh.
The best way to achieve and maintain healthy nails is through a well balanced diet, with proper amounts of protein, fresh fruits and vegetable. If this is not possible, then one may take supplements which include proper levels of Vitamin B12, C, Zinc and Iron. If one is just cursed with bad nails, perhaps by a genetic disposition, please do not let this deter you from making fine music on your beloved instrument. Many great players, such as Fernando Sor, had very short nails, and they turned out just fine. Others, such as Francisco Tárrega, went through periods where they used nails and then later disposed of them.
Whatever approach one takes, it should be the right one for them. It should be comfortable and never become the source of contentious debate. - jf