Just as a curiosity, where are the fuses for those additional ports?
Any recommendation for the wire size?
Again, the below is from my 68 Nova, and other models may very some but on mine, the BAT terminal tap is protected by the 20A fuse to the left. On the ACC terminal tap this is protected by the 10A fuse to the right. The IGN terminal tap is protected by the 20A fuse to the right, and the remaining tap labeled LPS is protected by the 4A fuse to the right.
While I do not have a good clear shot of the backside of the above fuse block and how the taps and fuse clips are tied together, below is from an older Chevelle.
If one is unsure as to what factory fuse is protecting what tap, you can do electrical checks with a meter.
As far as wire gauge, a fuse, fusible link, circuit breakers put in place to protect the wire, and there is a relationship between wire gauge and the amount of amperage it can handle without damage. In my way of doing things, a 30A fuse should be used on a 10 gauge or larger wire, a 20A fuse on a 12 gauge or larger wire, a 10A on a 16 gauge or larger and 5A on 18 gauge or larger.
Keep in mind too that the taps in the fuse block, even though they may be fused at let's say 20A, if 15A is already being used on that circuit fuse, technically you are limited to an additional 5A of draw out of the tap. If you were to try and pull an additional 15A through the 20A fuse and 15A was already being used by something else in the vehicle, then the 30A of draw would blow the 20A fuse. You just have to figure things out and not randomly start plugging things into those taps.