Hi, welcome ..
So if you are going to start with a 6"-12" purple ghost, maybe plan on field growing it first. Instead of planting it in a pot, plant it in the ground. I think there it will get bigger and fatter for your Bonsai start.
Like Bonsai Nut has mentioned, the PG has large leaves for Bonsai, but perhaps it is possible to smaller size with a technique called "Defoliation". I am not sure if that's possible (Even if you do that, that would be one of your last steps). I am also not sure how this plant will take some harsh treatments like "Trunk Chop" for thicker trunk and taper creation.
I plan to do what I suggested above with few of my Japanese maple grafts that I bought over the last few years. I have few that are growing from ground are actually looking like real bonsai while the ones I intended to train as bonsai are looking rather thin.
I actually don't know if that process will work well for Acer palmatum as much as it works for Acer buergeranum (some of the expert here will tell you that they wont) .. so it will be bit risky, but something to consider ...
(I am not suggesting you cannot use bigger pot, but based on my research, field growing will