Not really sure about snails, but a couple of months ago I emailed Waterlife direct asking which of their products were safe to use with shrimp. Their reply is posted below:
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Myxazin,Paragon and Octozin are safe to use in the tank with the shrimp. Protozin contains copper and as such can kill some shrimp.
regards,
WATERLIFE.
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Not sure if the copper also applies to snails .... my guess is that it probably does (but it is just a guess).
If in doubt you could try emailing them at info@waterlife.co.uk, they didnt take long to answer when I got in touch.