Can I file a small claims suit if I only know the defendents P.O. Box number and not actual Residential Street Number?

Yes, but it'll take some work. You'll need to know their actual address so you have them personally served with process of your claim.

You can get their actual physical address from the U.S. Post Office and serve the person at
that physical address. Fill out a 'Request for Change of Address' form (or one containing similar
content) and take it to the Post Office. If the address is old or invalid, you can have the Post Office "lockdown" the box until the boxholder provides a current physical address.

Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.

You can serve an individual defendant in one of three ways: personal service, substituted service, or service by publication.

The first two methods require knowledge of the defendant's address.

Service by publication is effected when you don't know the defenant's address, but requires a court order.

I would strongly urge you to consult immediately with a litigator skilled in this area of the law.

PLEASE NOTE THAT THIS RESPONSE SHOULD NOT BE CONSTRUED AS A LEGAL OPINION OR ADVISE, AND DOES NOT CREATE AN ATTORNEY-CLIENT RELATIONSHIP. IN ORDER TO RENDER A LEGAL OPINION OR ADVISE, THE RESPONDING ATTORNEY WOULD NEED FAR MORE INFORMATION THAN HAS BEEN PROVIDED, AND WOULD NEED TO BE RETAINED PURSUANT TO A WRITTEN FEE AGREEMENT.