This are the main rules:

1- All strings priced between 6 and 30 US$ are the same from a practical point of view. 
2- It is preferable to change 5 bucks strings every month than leave 20 US$ strings on for ages...

When the string are new, you must absolutely avoid using them in concert or recording. In case you must use new strings, pass the bass strings in 2000 grit sandpaper in order to remove the excessive abrasiveness that causes unnecessary string squeaking. Hold the sandpaper in one hand, pull the string from one extreme in the folded sheet of sandpaper. Do this 3-4 times for each bass string. 

STRING TENSION
Strings must be low or normal tension to allow for a sweet sound and beautiful vibratos. The higher tensions are only recommended for flamenco style playing or for people who uses extremely low actions. High tension strings should only be used by players who have a beautiful round sound or else the sound will be harsh and poignant. If your nails are thin or break and peel easily, use low tension strings. If you use high tension strings, you must use short fingernails or else the fingernail clipping against the nylon surface will be unbearable..

ACTION and string buzzing
The rule of thumb here is: it is preferable to play on a low action that may have a buzz here and there than on a buzz free high action. Most pros work around buzzes. A knowledge of the instrument is all it takes to work around buzzes. The best action should be kept between around 3.5mm at fret 12 (measure from the top of the fret to bottom of string). Some players find 3 mm to be perfect. This height will probably cause  a few buzzes, but the ease of play is worth it. You will have to adjust plucking strength on some notes. Most people who complain about difficulty in barring are probably using an action that is too high. Even when they are able to play the barre, the amount of strain will make the left hand tired in no time. Lower the action !

Scale lenght
Regarding 650mm or 640 mm guitar construction, keep the following rule at hand: If the distance between the tip of your thumb and the tip of your little finger is equal or less than 9 in. when fully stretched, use 640 mm (image below). In the many years teaching and playing the classical guitar and having worked closely with such great teachers as the Spaniard Jose Tomás and the Uruguayan Abel Carlevaro, it came clear to me that often hard to polish musical passages were not the fault of the player but a fault of the scale used for the fingerboard. Modern guitar construction is no longer dependent on scale length to produce a substantial volume. When we talk about scale length we are talking to the relative distance of the frets from one another... we are talking millimeters or fraction of a millimeter, but it is these fractions that make all the difference between flawless and sloppy technique....let alone the great frustration that derives from never being able to polish a given musical passage. 

Radiused fingerboard
The radiused fingerboard has been around the acoustic and the electric guitar since day one. The same is true for most instruments of the Stringed family of instruments (violas, violins cellos, bass). Radiusing (image above) makes barre and left hand stopping A LOT easier. The only drawback is that you will NEVER want to play a  flat fingerboard after that. If you are having your custom guitar built, I am all pro radiusing the fingerboard. The 1/16 radius is standard on all Bellucci guitars. The standard fingerboard is flat. When you relax the hand, you will notice that the natural shape is with a slight curve on all 5 fingers. Pros playing is about achieving to perform on the guitar  in as much a natural and relaxed manner. Whenever you part from these directives, you are making your path to perfection steeper. The radius is standards on the acoustic, electric and archtop (jazz) guitar and, if you look closely, you will notice that these players will use lot less energy than classical players and that their fingers are almost always curved.

Click here to see how to put strings on a classical guitar