I had a request for the Wilton Castle Kit for a cake for next weekend. She wants a 9" and 6". I noticed that the kit calls for a 10" and 6". Will the pillars fit on top of a 9" or do I have to do a 10" to get all of the pillars on there?

Place a 6" cake pan on top of a 9" cake pan and use the appropriate towers to see if they'll fit.

HTH

No, it won't work. It barely works with a 10 and 6. I've made three of those cakes and the first time I cried nonstop. The towers would not stay up (I didn't have the melted sugar candy to hold the towers to dowels). I pushed them into the cake and the 6" layer cracked into pieces. Many pieces.
Eventually, DH drilled the bottoms and I stuck the wooden dowels in.

It sounds like you haven't done one of these cakes yet...make sure you read ALL the directions a few days before you need to make the cake. And don't wait until all the towers are up to put the flowers on the towers. That's what it says in the directions, then you're left trying to maneuver around the standing towers. Yuck!
I hate that stupid cake.