Legal Question in Civil Litigation in California

If you're trying to get documents from his client or another party, you don't need a subpoena. You should use a request for production instead. The original would go to counsel for the party who has the documents. If there are other parties, you would serve their lawyers with copies.

You only need a subpoena if you're seeking materials from a non-party. And yes, you would have to serve a copies of the subpoena on counsel for the other parties.

Yes you have to serve the other parties. Furthermore, if you are requesting "consumer" records, then you have to serve the other party's attorney before serving the subpoena.

Opposing counsel only gets served if the opposing party is a "consumer" who must be given notice. If if is for trial and does not meet that category, it does not have to be served on opposing counsel. If it is for discovery purposes and not trial and there is going to be a deposition, then a deposition notice should be served on opposing counsel. I suggest consulting with an attorney for help with this, as the court would have to issue a subpoena if you are representing yourself and are not an attorney.