Exactly. There is not really set stocking limit for any size of tank, it depends on two main factors..
One of the main two things is the water quality. You should obviously have 0ppm ammonia and nitrite, but your nitrate reading is important when it comes to stocking a tank past the 1" per gallon rule.
What is the nitrate reading from your tap water? And what is it from your tank?
If your nitrate reading is under 100ppm in your tank, it is acceptable. I personally would never dream of having it higher than 40ppm, but this can be difficult if its high in your tap water. As long as your nitrate reading is under 100pm your tanks nitrate reading is less than 40ppm over your tap water reading, you can consider adding more fish.
The second thing is personal space, all fish, even schooling species like black neons need their own space to swim.
However, you shouldn't go over the 1" per gallon rule if your tank is less than six months old.
And if you do go over, you should over filter the tank.
Your tank is 'overstocked' already considering swimming space, for that many of those fish I would say a 15 uk gal would be minimum for them to thrive. Black neons can get to 2" and regular neons to ~1.7" give or take a little (pretty much depends on the quality of the neons, lots of neons have poor genetics due to inbreeding).
Adding a few more shrimp or some smaller fish like microrasboras wouldn't be a problem, just remember the rules above.
