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.

There are no ethical concerns here. You don't need to go into an interview blind about the project.

It is possible, of course, that people have been asked not to correspond with people in the pipeline other than officially, but that is up to them.

You can ask. But the tone of your mail will determine whether people think you are looking for information or for an advantage.

You can also ask after the interview, of course, but before you need to make a decision.

First I think there's an important point to mention, even if it doesn't answer the question: in theory the interview is a two way street, i.e. not only the PI wants to know if you're the right person for the job, you also want to know if the job (and the PI) are right for you. There would be nothing wrong with asking some details about the job during the interview, and the decision to take the job (should it be offered) doesn't have to be made immediately.

I have seen academic job ads where canvassing is explicitly prohibited, for example see these "instructions for candidates" from University College Cork:

Candidates should note that canvassing will disqualify and will result in their exlusion from the competition.

However I don't know if this kind of question would be construed as canvassing or not, and normally if this was the case at the institution where OP applies it should be mentioned clearly in the job ad.