I've done this a number of times... Periwinkles are fantastic glass-cleaners, but will climb down the tank, so you really need to check at least once a day!
They can cope with far higher temps than we get in our tanks; they will sit above the waterline in full sun in their native areas, and here, (Cyprus) that can be rock too hot to stand on bare-foot!
I've found that about thirty is ideal for a 50 gallon D/T, with a rate of attrition of about two a week; normally "escapees" that get eaten by the dogs!
I also use Babylonia, who do a decent job of sand-shifting, and I have a few serpent stars, all caught locally.
Because there's no coral in their local habitat, it's not likely to form part of their diet. If I see a nuisance alga in the tank, I will actively seek out grazers from similar algae in the rock pools... Nothing in the new tank yet though!
Crabs are another matter; local hermits will be used to taking those same snails' shells and think nothing of killing the snails!
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