If the target defendants have only SSI as income, that may impact collecting the judgment, but it won't impact suing for the damages.
To serve a defendant for whom you have only a P.O. box, you have to make a diligent search for the underlying address, and if you cannot find it, can apply for leave to serve the summons by publication.
If you want the whole $20,000, then yes, you must sue in limited jurisdiction superior court, not small claims.
The only way to find out whether an attorney will accept your case on a contingent fee is to take all the information and evidence you have when you meet with the attorney and see what he or she will agree to.