You'll find opinions will vary in this hobby for almost any question you ask. It does not mean any one answer is wrong. It's just that success will be measured in different ways by different people. Keep in mind the higher the bio-load, the more maintenance you'll probably need to do on the tank. Your fish look fairly small right now. They will grow. You'll want to take into account possible aggression from over stocking, especially as fish grow and begin to fight over available space. With that said, some people like to add fish (within reason) and rehome bullies as they get larger. Many others choose to stock based on the full grown sizes of the fish they select.
I don't think there is a fast, hard rule of thumb. I think stocking depends more on the type of fish you have than inches of fish per gallon. For example, your tangs will want a lot of swimming room without bumping into each other when they are bigger. They will grow 12" and more based on the species list posted. The goby will keep to the bottom of the tank; the clowns will host a specific area of the tank and likely protect it from invaders when they get bigger, and the dartfish will keep to the rock crevices for the most part. So in a nutshell, adding new fish will depend on the type of fish you want to add, their full grown sizes, their ability to get along with the type of fish already in your tank, your diligence in keeping up with the maintenance, and your ability to quickly settle any aggression issues that come up as the fish grow.
Another thing to keep in mind is some fish grow much faster than others. For example, my Hippo tang grows very slow compared to my banner fish which have quadrupled plus in size in just a few years. I got the banner fish as tiny quarter sized fish. One outgrew the other and is now the largest fish in the tank. Some fish grow even faster. For example, a porcupine puffer would need its own 180 gal tank within one year. I like to try to take into account all the above and then deal with issues as they crop up. For example, as my parrot fish grew it became a bully. My kole tang started out a bully and never stopped no matter what strategies I tried to deflate its aggression, so I rehomed them both.
You already have a lot of fish that will grow large. I would think if you add some med to small wrasses, hawkfish, or other semi-aggressive fish you would probably have fewer problems than if you try to add more large tangs or other larger fish. If you do decide to get the foxface, it is almost always left alone due to it's poisonous spines. I put a two inch foxface in with my larger 6" - 8" fish and it was left alone by all.
You don't say what kind of snails that died, how you acclimated them, or include the temperature of your tank, so their demise could be from a number of things: Water quality issues, too quick acclimation procedures (inverts are very sensitive to PH and salinity changes), old age, and/or the purchase of cold water snails sold unbeknownst to the buyer as tropical snails.
