Anyway, symbol-set, symbol-assign, symbol redefinitions, then specific symbol-hide or -show with layout file copys somewhere in the middle seems like a sensible rule of thumb.
Yes. You may note that "copy" can be used either before or after, depending on whether you want to copy a set of defaults, or copy a specific configuration for symbols. This does mean that my "copies first" method is not always appropriate, and it depends on the specific intention of the copy. Since you will generally know exactly what you are copying, you can presumably work out the approach that works for your case. Just treat "copy" as being replaced with the contents of the layout that you are copying. If your layout and the layout you are copying both try to set a whole load of symbol-set, symbol-assign, symbol-show/-hide etc. then you could be in a world of hurt.