Yes they are made to fit behind the faceplate.
There are a total of 14 -8 Outlets- 4 Switch and 2 Décor Switches.
However Frost King themselves do offer a wider variety of these insulating kits. Check their website. Home Depot I usually can order online products not offered in store but for some reason they have a limited selection of these. I also checked all the Frost King recommended retailers but they had no bigger selection than Home Depot. There was only 1 website I could find what your looking for. It is my "Prime" favorite to use when I can't find what I am looking for at Home Depot.
No, they are not made for octagon boxes.
Yes this can be used behind the cover plate of the Dimmer Switch.
you will have to cut off a side off the regular switch place insulators and than you can make it you're self.