I like that you are being patient with buying fish to ensure the environment is stable, that is a good idea.
Shrimp do shed an outer shell when growing so you are correct the transparent outer you found would likely have been this so all good.
I’m a bit like you with snails too, I don’t like killing them as long as they don’t become an infestation. I like seeing them in a tank, some people don’t.
A quarantine tank doesn’t have to be a permanent fixture, a plastic storage container can be used and removed as necessary. Personally I don’t use a quarantine tank, I try to buy good stock but really that is a bit of a lottery
The tank is looking nice and green by the way
Cheerio
Ady