you could double up 2 pairs in series, then you amp only thinks you have 4 pairs of speakers.

If you have a 4 channel amp you can run the 4 6x9's of two channels and bridge the other 2 to run the 4 8 inch subs.

Your head unit will not burn. It might overheat, but it will just shut down the amp until it cools off. It you like the way it sounds let it rip. You can always change it later.