i have a 55 gal, i have 3 cleaner shrimp 1 blood shrimp, 1 coral banded shrimp, 20 hermits of all colors, 30 varius big snails... many of thoes small lil buggers though
i dont remember where i heard this but i know it was reliable, u should have around 1 cleaner per gallon. this means there is enough material in the tank for the clean up crew to continue to live on but will also keep the tank relatively clean.
i know that when i purchase my clean up crew i will stick by this rule.
I don't think there is a "rule of thumb" that could easily be applied. If you have a 55 that is fairly new...limited amount of LR...few fish/corals...and not much algae growth then possible a few snails/crabs...and wait before adding shrimp.
But...in a 55 with lots of well established LR...mature with stable water chemistry...numerous fish/corals...heck at one time my 55 had 40-50 snails...20 each redleg and blue leg hermits...4-5 emeralds...8 assorted shrimp...a sallylightfoot...and multitudes of worms/pods etc.
So...I think it really depends on the demand of the particular system and can't be determined by a "rule of thumb".