RX8 takes 6x8 front and rear. The only 6x8s I see are 5x7 and a installer told me its the same thing?? Will these fit in the car. The site says they need an adaptor to fit.

http://www.cardomain.com/item/INFREF6802CF?vq_id=none

 

No - the rears are 6x9.

They are not the same thing, but many 5x7 speakers do have the mounting holes for a 6x8 location as well. However, they are certainly smaller than the 6x8 - on the other hand, you have more choices with 5x7.
I'd still stick to the 6x8 size though - or better yet, use an adapter and install a 6 1/2.