I'm not sure how it would work for small firms but in my experience (previous application cycle) I don't see anything wrong with that. I did it for a large American firm and it was clear trainees knew how much information they were allowed to divulge to keep the process fair.I have an interview with a small firm. Is it okay to reach out to a trainee and ask about the firm over a phone call or would it be used as trying to get an unfair advantage?
As long as you don't ask extremely specific questions, I don't see anything wrong with it.