Ok - my best friend (soon to be former! 
) packed and moved my 9" pans I let her borrow and just sent them on a moving van across three states. I have a wedding next weekend! I usually do a 6" 9" 12" - would 8" look okay? Anyone think someone will notice? 
It would look like the bottom 3 tiers of this cake, if it's going to be cake-on-cake:
http://www.cakecentral.com/modules.php?name=gallery&file=displayimage&pid=94152
If you are going to put pillars between the 6 and the 8, it will look great, because the space between the tiers will minimize the closeness in size between the 2, like this one (14/10/8 ): http://www.cakecentral.com/modules.php?name=gallery&file=displayimage&pid=113263
Could you cut/size down your 12"? May need to charge her for buying a couple new ones! 
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