Through my personal observations, I use 1 Astrea per 2 gallons of water. Everyone's system is different though.
I have a 180 gallon tank and have about 100 Astrea's in there (200 Cerith's), and my rocks are clean.
-Astreas are great at harvesting algae.
-Ceriths are great at harvesting other algae - and - astrea poop.
-Ceriths will make the astrea poop easier for you to harvest with a skimmer.
I have scarlets, blue legs, every snail commonly in aquaria, and peppermint shrimp, with no detriment to candy cane, frogspawn, pocillaria, acropora, etc. A scarlet just traipsed over the base of a bali green slimer a moment ago, and provoked nothing from the coral but indifference. Those little feet are light and accurate.
I myself and others believe hermits are just for show, doing little to clean your tank. Snails do the cleaning.
A test: Start two 55g tanks with 100 lbs live rock, each with 2 250 watt MH lights, filtered similarly. Fill one with 50 hermits and one with 50 snails. See which tank stays cleaner.
++++As for where to get livestock you put in your tank++++
The cleanest, best run local fish store that has happy fish and good customer relations with repete customers are the best place. You buy what you see.... healthy feeding motile livestock.
Ceriths and astreas. Turbos get larger and can dislodge things. I believe in leaving reef tanks, crab free. Just my opinion. They dont do anything that special anyways that the snails dont do. I have used scarletts, but they dont seem to really work like the snails, so why risk it, but if i did have to put a crap the scarletts.