The stocking rule is very often subjective to other factors beyond just the size and quantity of the fish in the tank. Water changes, type of fish, tank mates, keeper experience, potential adult size are just some of the factors that should be used in determining reasonable stocking.
I've been keeping fish for about 25 years now, and I consider a tank overstocked when the crowding creates aggression or an unhealthy social environment, fish are being stunted due to inadequate water quality (basically the fish grow to the size of the tank myth), the fish are physically or ethically too large for the tank, or the biological filtration of the tank can't keep up with the nutrient supply created by feeding and waste. In your case, I suspect none of these conditions are anywhere close to existing, assuming you have a reasonably sufficient maintenance routine.
The 1" per gallon rule is a really broad, and IMO a really bad guideline. In your case, tetras are extremely small bodied fish, so if you are using that guideline, you can safely exceed it by quite a bit. Conversely, if you are stocking a large bodied fish, fish with very high metabolism, or a fish that are very messy, the rule can easily come up short.
Also, do not count snails and other inverts into the stocking equation unless they are sizable enough to be a major nutrient contributor to the tank. Snails and small shrimp should be excluded. If you had something like crayfish or other larger inverts, I would factor them in, but something more like a fraction of the 1" per gallon rule.