For starters, emailing an alum at a firm you want to work at isnt exactly random. I've emailed plenty of alums asking about their experience at the firm and what it's like to work in that particular legal market. If they work in the same area of law Im interested, I ask some pointed questions in that regard too.
If youre applying to a firm with tons of alums, then you may want to limit it to ones working in the field of your interest. However, you could always mention that they are free to forward along your information to any other alum who may be of interest.