As far as I know, you don't count most inverts in the fish rule. Most inverts, i.e. crabs, snails, urchins, stars, etc., are cleaners in the aquarium. They are not producing more waste than they are consuming, so they don't count. Fish produce the most waste in the aquarium, that is why they are counted.
That said, you don't want to go off and throw 100 blue-legs in a 10g aquarium because that is just too much of a load(I've done it cause I had nowhere else to put them). There are recommended quanities of the different inverts per gallon that should be followed, but can be over-loaded by a little. I have about 3 astrea snails per gallon in my biggest tank and it is fine, and there is practically no algae.
Basically, just don't overload the system with more than it can handle. I wouldn't add more than 1-2/gallon of anything, except maybe certain snails and blue-leg hermits. Also, don't add more inverts than there is food for, don't want them to starve.