Hi, I am very new to this cake stuff.
Anything more adventurous then a 9x13 betty crocker mix with general basic frosting has never been attempted by me.
I am making the wilton's castle teired cake. Is it ok if I use a double stack(as in two) 9 rounds with a regular rectangle loaf cut in half on top?

Welcome to CC

You can mix and match whatever shapes you'd like. Just make sure that you have support if you're stacking your cake over 4 or 5 inches.