arrow crabs are bad with other fish, I believe they can eat fish but either way they aren't good unless you want a tank for them
I don't know if you want that many shrimp but it's 1per5gallons I believe so thats about 20ltrs
Agreed with the hermits, best not to mix them I dont think and sometimes they can get grumpy so you may want to leave them out all together
sea slug's are incredibly hard to keep alive so don't get one, along with being sucked up by powerheads and nuking your tank
snails=cerith, astrea, turbo's, look back a page or two there was a marine topic of a week about them. Don't get margarita snails unless you keep your tank cool 74 or less, they don't like hot temperatures