Should I use round or rectangular electrical boxes for stubbing exterior security cameras?
All the ones I've seen seem to be sized for installation on a light box.  You want to use as shallow a box as you can get away with so you don't decrease the wall insulation.  If you use the really shallow ones, you might be able to surface mount them on the sheathing over the tyvek.

Do you even need a box?  If the camera is PoE, you're running low voltage and tiny currents.  

I think your best bet is to look up some of the cameras, then go to the Mfgs website, and read the installation guide. 

Don't put them all at 8 feet.  That's really easy to hit with a spray can of paint.  Mind you:  Any camera is subject to a paint ball gun

You can often find dummy cameras.  These can give a bit of false security while you are waiting to afford the others, and they can mark the places where your cable comes through the wall.

Place one camera where it can read the license plate, both coming and going.

Place at least one camera where it can get a good face shot of someone leaving the house.  These cameras are much more difficult to scout, and most thieves aren't actively looking for surveillance on their way out.