DISCLAIMER: Not everyone agrees with my views on fishkeeping or stocking and it would be prudent to wait for more people to respond than just me before making any decisions.
Okay, first order of business: Aquarium salt is not very beneficial long-term. It can be useful when first stocking a tank or when medicating a tank, but the bottom line is that freshwater fish are just that--freshwater fish that should have freshwater. Granted, the salt, in moderation probably won't result in any serious complications, but it won't help either since the concentration is so low. Another thing to consider is that most of the freshwater fish that have lived to record ages did so without many aditives in the tank, including salt.
As to whether or not the tank is overcrowded/overstocked, that is going to depend on who you ask and what you define as being ideal. By many standards I have seen (believe me, I've researched many), your tank is definitely overstocked. Your tank exceeds the inch-per-gallon rule (use the adult length of your fish when making calculations) and it also exceeds the twelve square inches per inch rule. These are the two most common stocking guidelines. However, my one area rule, Joseph Levine's (biologist), your tank is okay. No less, all of the above rules do not account for many factors like tank cleanliness, adequate filtration, compatability of the fish, etc.
Some aquarists prefer to judge the suitability of a system by measuring the nitrogen compound content (ammonia, nitrite, and nitrate). They essentially believe that so long as the fish get along with each other and the nitrogen levels are low, that the system is just fine. One of the problems with this stocking view is that it can push the aquarium to the point where anything that goes wrong can have drastic consequences. For example, if the power goes out on a tank whose nitrogen levels are kept in place by frequent water changes and high levels of filtration, then the aquarium may not last very long on its own. Simply, if you push an aquarium system to its maxium, then the margin of error will be fairly small.
I, on the other hand, prefer to use a combination of factors to assess whether or not an aquarium is overstocked/overcrowded. You can check out my views on stocking on my stocking website, http:/sites.google.com/site/moashowmanyfish
. But the short version is that I do not think that your aquarium is necessarily overstocked. Also, I used to breed guppies and I've had more than 100 in a 10-gallon tank before, but with daily water changes and a back-up generator.