i am sorry to say but you are mistaken.
nuestro/a/os/as works the same as a pronoun and as an adective
the pronoun or adjective have to agree with the subject:
-nuestro gato es negro
-nuestra gata es negra
-nuestros gatos son negros
-nuestras gatas son negras
-el gato negro es nuestro
-la gata negra es nuestra
-los gatos negros son nuestros
-las gatas negras son nuestras
'"Ours" is the first person plural possessive pronoun - it replaces "our" + noun."' http://www.elearnenglishlanguage.com/difficulties/ours.html
When the accent is used to show the emphasis falls on the last syllable (like ión nouns) and it's made plural by adding another syllable (es), then the plural noun now has the emphasis naturally fall on the syllable that previously needed the accent and it doesn't need one anymore. condición > condiciones. But that's the only rule - take out the accent if it's no longer necessary.