What fits best, crud blades rubbed like hell
Well...the CAAD8 is a race-orientated frame so it isn't really designed to have mudguards fitted, hence the rather small clearances around the BB area and under the brakes. Therefore, any mudguard you fit is going to be something of a compromise or 'adaptation'.
You may therefore want to consider doing what I did to an old Specialized Allez a few years ago when in a similar situation:
Establish whether or not a set of aluminium Salmon mudguards will fit under the brakes.
Assuming that they will, purchase a set of the aforementioned guards and, if you find that you don't have any means of attatching them behind the BB shell, 'doctor' the mudguards to fit by trimming a few inches off the BB end of the mudguard and then attaching this end to the seat-tube of the bike a few inches above the BB. When I did this I used electricians tape to protect the frame and then attached the mudguard by using a couple of zip-ties and plenty more electricans tape.
It's not a pretty solution and on your CAAD8 you'll have to be very careful not to allow the mudguard any movement as any rattling and rubbing will likely damage the frame.
But to raise another issue, from the forks I've seen fitted to CAAD8s from a google image search, you'll have to use P-Clips to attach a Salmon mudguard to the fork, which again may result in damage to the fork.
I rode a caad10 around Norway earlier this year with some sks race blade longs fitted. Seemed almost as solid as proper guards, and no issues with rubbing. I was so impressed with them i came home and binned my cruds and got a set for my training bike.