I like it, the only concern is where would the sata power be seen? I like the white stripes on the 24 pin atx, but the sata power is a hit or miss kind of place to also have white. I think it could look nice, but I think it might be better to not draw attention to it and leave the middle wire black.
Edited to actually answer your posts questions
Personally I refer to this chart. https://www.powerstream.com/Wire_Size.htm
On other posts is seems the heaviest wire used is 18 gauge, but they can use smaller, I would be comfortable with 19 gauge, if you really want to make sure you can look up your psu specs and double check no wire experiences that high of an amperage.
The source I linked has really conservative value for max power transmission amperage, almost a 50% value