Can I serve interrogatories on nonparties by certified mail in NY State Supreme Court?

I'm not sure you can serve them on non-parties. See CPLR sections 3120, 3122.

Under New York State law, you may serve the interrogatories by First Class U.S. Mail (CPLR 2103) to establish valid service, since you're concerned about money. You can draft an Affidavit of Service for each non-party and complete them once you have made service of the interrogatories in case you are asked to produce proof of service.

Dear Rome Plaintiff: You are allowed to conduct an examination by oral questions of "any person" if the deposition notice is served as provided at CPLR 3107. The use of an written Interrogatories is between parties as provided at CPLR 3130.

I would agree with Mr. Smollens. You can not use a Demand for Interrogatories on a Non-Party. Your recourse is to serve a Subpoena and Notice of Non-Party Deposition (containing the requisite language) on that person, and take an examination asking your questions, using a "court reporting" service. The subpoena and notice of non-party deposition has to be served as if it were a summons and complaint, meaning using a process server, and has to include the requisite witness appearance fee (the failure to pay the fee will nullify the subpoena). Since you are acting Pro Se, you can not personally issue the subpoena and will have to get it signed by the judge to whom your case was assigned. Copies of any subpoena and notice has to be served on your adversaries, who have the right to be present and also ask questions. the failure to do that will prevent you from using the transcript at the time of trial. Be aware, that if the non-party is outside the State of New York, as the subpoena has no force and affect beyond the State's boundaries (then you have to go through a process to get what is called "Letters Rogatory").