That would be about right. Low voltage, low current...you could probably even go a step smaller, but if you have the room, and it fits in the harness, why not leave it at 18.
I understand. You can always go bigger, but if there isn't a significant current draw, 18 is probably overkill. Just put in the 18 if you have the room. You can always go bigger; just not smaller. There isn't any circuit in the micro that draws significant current. Just use stranded wire, not solid.
