Legal Question in Landlord & Tenant Law in California

Sounds like a breach of contract. Careful, though, Santa Cruz may have laws that allow for a waiver of contractual terms like payment dates and the monthly rent. If you've continued to accept rent lower than the contract amount, then you may be deemed to have modified the contract, or at least to have accepted rent passed the due date, thereby changing the terms of the contract to allow payment at a date later than the first of the month. Hire local attorney to consult with. It doesn't cost must and is worth it. Good luck.

Yes. You can give them a 3-Day Notice to Pay or Quit. If they either don't vacate or pay the rent, you can file an unlawful detainer action. You should hire local counsel to handle this matter. While you many people "do it themselves," the eviction process is driven entirely by statute, and your failure to give adequate notices or follow the statutory process could result in you having to start all over again. The cost for hiring an attorney is somewhat dictated by local practice, but most attorneys will charge a minimal flat fee (plus costs) for an "uncontested" (meaning the tenant does not file a responsive pleading); if the tenant does file a responsive pleading, most attorneys then charge an hourly fee for the time they spend on the case after the responsive pleading is filed.

I believe Santa Cruz has rent control so you need to read through those rules. Tell them that they have to pay the full amount agreed to. if they have no real assets, then even with a judgment you probably will not be ale to collect anything from them, so try to get them to agree to move in return for your not suing them.