Is it possible to put a 6 inch fake tier on top of  9in and 12in cake tiers? I've done fake tiers before but usually the bottom tier. 

Any idea? 

AThose are so light, I don't see a problem. If you're really concerned you could dowel the top tier Too, But I wouldn't bother with it

Brilliant, thanks!

A top tier doesn't need to be doweled, but you could skewer it to the 9" if you like.