Regarding the question of adding in or changing of existing looms on a bike: the advice given in previous threads applies. You need a particular size of wire for a particular amperage over a given distance. There have been few cases where a conductor is too large for the application, due to the nature of electron flow. However the fuse or breaker must be matched to the ampacity of the device or else it offers no protection at all. Amps x Volts = Watts and dividing Watts by Volts tells you the Amps and required fuse/breaker rating.
This means that if you have an 18 gauge wire going to a marker light as original equipment you don't tap into that, just because it has power to it, to install a pair of 100 Watt Hella lights.
The same obviously goes for the fuses on a given circuit. They're sized for the length of run and the amperage load of the device, period, nothing else! In many cases I've changed from fuses to commonly available circuit breakers yet with the same amp rating.
There are 'Mains' circuits in a bike that are heavier gauge than 'Auxiliary' circuits. If there aren't, and they're becoming more of a rarity, MAKE YOUR OWN!
Where you have existing wiring either in a loom or separately, you need to trace it by hand or by diagram to find out what all is connected, what the source fuse rating is, and the amperage of what you wish to add. In some cases, if you're planning to add a lot of doo-dads [my, how we do love those doo-dads!] it's best to go back to the main power panel or fuse box and identify the main circuit, then add a new dedicated main conductor for all the new stuff.
You run a new wire of say 12Ga from main panel to dashboard and then you can install inline fuses for all the sub circuits off that along the way. These may be lighter gauge wire than the main, specific to the current draw of the device, but never heavier gauge wire or fuses. The separate fuses or breakers for each new device if properly sized will blow before the whole circuit overheats and causes disaster. The new main wire circuit must be of sufficient gauge to handle the total load of all added devices at once, so note the current draw on this circuit somewhere as you build. There is the straw that broke the camel's back syndrome.
This way you have an independent circuit that doesn't put any extra loads on existing wiring and it's pretty simple to run one new wire. If possible a new color can save you lots of fiddling or headaches later, always knowing that's the new heavyweight 'Main'.
Same with the hack: one power source over the side, then sub-fuses at the different devices for microwave, colored TV, DVD player, air conditioner, ice maker and dish washer.
One thing that causes more problems more often than anything else on vehicle electrical, especially bikes, is POOR GROUNDS! Corrosion is so common with bikes and it wreaks havoc on grounds. All chassis grounds must be to clean bare metal and an anti-corrosive, like dielectric grease or others should be used. Vaseline or chassis grease will do in a pinch. Grounds must also be rated to the current load in a given circuit.