You can use it as an evidence of their monthly rent.
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You probably could enter them, but it's irrelevant. UD actions are highly statute-driven. Tenants either pay the rent, or they don't; if they don't, they have to leave (after proper notice and an opportunity to cure).
No, you cannot request judicial notice to admit into evidence the income and expense declarations filed in a separate action. Such declarations could possibly used for impeachment, but you won't be able to just ask the court to take judicial notice of the truth of the matters stated in the declarations.
Frank W. Chen has been licensed to practice law in California since 1988. The information presented here is general in nature and is not intended, nor should be construed, as legal advice for a particular case. This Avvo.com posting does not create any attorney-client relationship with the author. For specific advice about your particular situation, please consult with your own attorney.