I dont quarantine snails or crabs, but then again i dont quarantine anything. Never trust a crab! But small hermits can be fine, always stay away from crabs with pointy sharp claws, i think that means they are more meat eaters vs stubbier claws being more meant for algae. I think ive heard arrow crabs are fine because they have small claws, but im not entirely sure on that one. I think by ugly, he means all of the random blooms you may encounter. For me, after setting up new tanks it generally goes diatoms<pods<green algae<brown algae<sometimes cyano. I think that happens to most people, hopefully its not just me lol