I haven't written a letter to anyone in years, so I don't know how to answer those questions. Otherwise, these days, any email I send around the university is 'Dear [first name]', whether it's to a fellow, a member of admin staff or another student I don't know; it's very rare for anyone to want it another way. But I also send very few unsolicited emails, so I'm mostly following how everyone signs their name when they write to me.