Re: Disconnecting battery question
If you disconnect the batteries, there will be no power in the wires so you can do the wiring changes that you need without worrying about getting shocked. This is with the assumption that you are not connected to any other electrical system. (shore power like 120volt ac)
There are no capacitors or other components in a typical boat system that can shock you when the batteries are disconnected. The comments above about being careful when wearing a metal watch or ring are valid concerns while the power is connected so you may want to remove those items when working or some will tape over them to prevent them from making electrical contact. Once the batteries are disconnected this will no longer be a problem. During your project, making a diagram of the wiring system identifying wire colors and putting labels on may help a lot in the future.
A rule of thumb is "when in doubt on what size wire to use, go to the bigger wire. Too small a wire will not be able to handle the energy it is being required to carry and will get hot. This can cause some serious problems.
Good luck with your project
Tom
