I'm not an electrician, but I don't see the harm in using a wire that's slightly over-specced for the task. Especially for high current items like starter motors and headlights. The only problem comes when you have a lot of wiring in a small space, such as behind the dash and under the fuse boxes where it can be very difficult getting everything to fit if it is all, say, 6 square millimetres.
Many years ago when we had a Series 2 Forward Control it was a nightmare because someone had helpfully rewired the whole thing with red wires, so it was very hard to trace anything and the wiring diagrams in the manuals were more or less meaningless. So it's well worth trying to stick to the original colour codings.