My personal opinion is a 650 scale is perfect because I think it is the ultimate in getting the traditional sound. A longer scale as well as a shorter scale will effect volume, tone, etc. Maybe ever so slightly, but still has an effect which may or may not be a players first choice. For example the tension on a longer scale is less than a shorter scale. While the pitch is the same, the characteristic are different. For extremes compare a 630 to a 670. The 630 has less presence of tone and the 670 has lots of presence but in my opinion is somewhat mask in the trebles. To explain the term presence it means to me to expand the reproductive frequencies of the guitar beyond what the lowest and highest note used are. Making the body larger, deeper, etc will tend to favor bass and dampen trebles. Having had a deep bodied guitar at 655 scale, I can tell you the trebles were slightly masked or dulled by the size of the body. They were loud and had very nice tone, but not as nice a trebles or bass growl as I like. The bass was deep but more "boomy" in my opinion. Making the body to small the guitar becomes to "tinny" and shallow. Putting a longer or shorter scale on a body like Santos, Torres, Barbero, etc., will have a very minute effect on the presence, tone, and volume to the ear, but I find the big difference is in the feel of how the strings can be manipulated which may cause the player to not play to their capabilities. I think a 640 open would feel alright. but with cejilla at 2nd or higher would be a bit stiff or tight feeling to the fingers. Maybe ligatos would be not so pronounced, etc.
Because string tension is less you might find a longer scale easier to press down the strings, but the reaches are longer. All my guitars are now 650 as I like the idea of having them all the same. I play 80% of everything with cejilla at the 2nd fret. I like the feel and sound there and most of what I play (siguiriyas, soleares, alegrias, bulerias, tango, fandango, sevillanas) never goes higher than the 12th fret with cejilla on 2nd. Open I play Malaguena, Guijiras, Taranta, Tangranta (my own palo not real flamenco), etc., and do a lot of practice octaves and scales there. Occasionally a Tango or Farruca if I want to show the full range of the guitar.
I find 640 a bit short for using the cejilla. I think 650 scale would have the best resale as it is standard and used by both men and women and has the best over all traditional characteristics. It is a very personal choice. Many classical players with small hands are going to 640 and find it to be more comfortable.