Help with installation wire type and gauge

Bigger is better; to a point.
Wire gauge will have two major criteria.
It will need to be large enough to carry the current and not so small as to be fragile.
14 AWG with a 20 amp fuse for the power wire cable would be a good choice.
Nearly anything will work for the Mag Switches and Tach Signal, but anything smaller than 18 AWG becomes a little too delicate for the environment that it will be in.

Mil-spec wire will be your call.
Just be sure to fuse the wires at the source end with a fuse that is appropriate for the wire it is protecting.
Assume one day it will chafe to ground and short out.  Maybe not in your lifetime, but assume it will happen eventually.
18 AWG can handle 10 Amps but a 5 amp fuse might be more conservative.
The Mag and Tach wires are not power cables and will not need a separate fuse as the ignition and tach circuits should already be fused.

Thank you.  That is what I needed.