Not unless you drill a hole to fit, but then it might not meet code. I'd use a metal box with knockouts to match the size of the conduit.
The short answer is yes, but I've heard of some areas not allowing old work plastic boxes to be used in a commercial setting which makes no sense. You can contact the electrical inspector in your jurisdiction for a 100% answer. I have used them with no problems.
As shown in the picture of this box, the part sticking up at the top of the box is an ear that pulls in against your cabinet wall when the screw for the ear is tightened.
No, I believe that in order for those "ears" to work, they have to have a special hole for the screw and a recess built in to the box to allow you to slip it into the wall before you turn them to fasten.
You don't need a template. You just hold the box up to the wall and trace it
Cutout needed for the batch of boxes I got is 3 3/4" x 2 1/4".
It will be snug, but most GFCIs should fit.