I'd say whatever floats your boat. I have the high mounted brake light with brake light spoiler and I added a roof line brake light too. It's excessive, but I was bored and felt like doing it and like it.
Eventually I want to get some carpet to match the rear deck so I can remove the OEM high mounted brake light and just have the roof line brake light.
As for the wire splicing of different gauges, I say it is okay. But I would make sure to use a gauge appropriate for the electrical component your connecting to. I also avoid splicing in a larger gauge wire into a small gauge source wire.