Is it ethical to reach out to other postdocs about the research project before the postdoc interview?
For every job I've contemplated I have gone out and done what I consider to be the necessary due diligence. If you have connections, use them. If it involves cold-calling a person or two to ask questions, do it.
Fun story - I was considering moving from technical staff to management and an interesting position opened up. My due diligence consisted of answering two questions for myself. (1) Did I really want to be a manager, and (2) was this position the right position to step into management. In the course of a week I talked to about 10 different people, some staff, some managers, some trusted confidants. I decided the answers to (1) and (2) were affirmative, so I applied.
The day for the interview arrived, and I walked into a room with 6 people, the interview committee. I had talked to every single one during my process. Not surprisingly, the interview went very well.
So, yes, go and ask. There are no ethical issues at all.