Well, sand dollars are reef safe.
Are they just in a QT or do you have a bare bottom tank w/o sand? If it's bare bottom, you're not going to be able to keep the sand dollar, and IMO, since you didn't know what it was, you probably can't care for it properly.
The hermits, they're huge, I wouldn't risk it. The snails are fine, I have the same ones in my tank. The sea apple, well, they're reef safe, but I suggest reading up on them a lot if you decide to keep it.