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In Cumpiano's book he has the back ladder bracing at 4", 7.25", 10.75" and 14.75" from the front edge of the back. How would this positioning change if making a smaller bodied guitar? Should there always be a brace across the waist? Should another be across the widest part of the lower bout?

Back bracing is pretty variable. I have an old Martin size 1 (parlor) that uses 5 back braces. Many classicals only use 3, and a number of large bodied guitars use 4. On some guitars the braces across the lower bout are wide and low, while others use tall braces throughout.
If you have a plan, you might stick to that, if not try to copy from an existing guitar of similar size. Or you can just "wing it".
I see back bracing as doing two things - supporting the arch of the back and bringing the resonance of the back into a range that works well with the top.
Making the braces that span the lower bout wide and low I think allows the back to move (flatten out) during low humidity conditions, reducing the tendency to crack. I think they also add weight without adding too much stiffness, which helps adjust the tap tone of the back.
I think a good back bracing scheme can help make a guitar great, but that a poor one won't necessarily keep a guitar from being good.

All the dimensions are proportional, so you would reduce the distance between the braces by the same percentage that you decreased the body size by, IE ten percent or however much smaller the body is.
You would also reduce the back, sides, top, and brace dimensions also by the same percentage, to keep the minimally adequate structure principle at work in Cumpiano's build method consistent. The smaller body is actually a slightly stiffer structure and therefore can be constructed lighter, materially.

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