"I am working with twelve employees."
Translation:Pracuję z dwunastoma pracownikami.
Numbers also declinate with cases. „Dwunastoma” is the instrumental case of „dwanaście”.
The rules you provided state that 12 uses genitive. So does the "z" using instrumental supercede the need for genitive after the number? This is so confusing!
The rules you mention only apply when you would normally use nominative or accusative. You can say that all the other cases are immune to that change.
Thanks for getting back to me, Vengir. My confusion arises because in the tables in the book (Polish Verbs and Essentials of Grammer / Swan) it gives both forms - -oma and -u forms as optional for instrumental for some reason, also stating "The instrumental forms in-oma are optional, alongside forms in -u" but I will make a point of not using the - u form from now on. The book wasn't written by a Polish native by the looks if things.