So this is more of a personal preference than anything. But I have a tendency to steer away from using connectors to join wires. I’ll put small heat shrink on, then if the wires are to be grouped together, I’ll put a bigger heat shrink over the bundle. I will then solder the wires together. Then slide the single heat shrink over, heat it up, then slide the big one over the bundle and shrink it. If it’s going to be exposed to the elements, then make sure you get sealing heat shrink and split tubing.
As for gauge/draw/length there are tons of charts and calculators you can use. Such as the following:
As for connectors, for those I’ll solder them as well if possible. This eliminates the possibility of them pulling out of a ‘crimp’ connector. Also, heat shrink those to create strain relief.