I'm making my own b-day cake, so no biggie if it fails. : )  I need about 9 oz of cake flour, and I only have 6. Would I be better off subbing all-purpose flour or cake flour for the last 3 oz of cake flour? TIA!

It have subbed all purpose flour for cake flour many times. For every cup of cake flour, I use 2 cups of all purpose flour minus 2 tbs. Then replace those 2 tbs with cornstarch. I have never done a mix of cake and ap so not sure how that would turn out for you.