the wiring diagram in the post above with the ford starter relay and the extra ignition wire is so the coil gets power when the starter is used. the coil will get a full 12 volts. some systems cut the coil power down through a ballast resistor during normal operation and this is to extend ign coil life. during start up these systems use that ign wire to feed full power to the coil for a better spark during the start up cycle. you should check the wiring diagram for the coil style you are using to see if it supposed to get battery power during the normal run cycle or run a ballast resistor during the normal run cycle. newer style Fords would simply not connect a wire to that terminal but still use the same solenoid.
for the different gauge of wire you are using it might be to your advantage to invest in a hammer style crimper for the larger gauge crimp on ends. like battery terminals etc. also some different colored shrink tube to easily denote the positive cables and the negative cables. most places that will make a cable for you use these tools of some sort and the guy doing it is no smarter than the rest of us. if he learned it you can too. try a starter-alternator rebuild shop for larger diameter cables, they usually stock the stuff and are reasonable in their pricing for building custom sized/length battery cables.
for the ignition switch it sort of depends on the system you want to run. the switch with no starter position are for the vehicles that still use the "step on the starter" pedal or a separate push button starter switch. normally the park lights, cig plug (sometimes), brake lights and 4 way flashers get power all the time with the ignition on or off. accy that draw less amps and will tend to not kill the battery in a few minutes, like stereo, get power in the accy mode, everything gets power in the on mode. most stuff is shut down for the start mode except for the ign system, fuel pump and starter, just what it actually needs to run so no other circuits will steal battery power away from the starter.
for me, I use welding cable for the big battery cables because it has more strands than most battery cables do so it is more flexible. if running a long cable from, say, the rear of a vehicle to the front sometimes the stiffer wire is better because it doesn't sag so easily. i use copper crimp on lugs, I have a hammer style crimper for the big stuff (that can also be used in a vice if you have 3 arms) but have also used the crimpers that look like a set of bolt cutters. both give decent results. I worked with an old guy who, when done the hammer crimp, would usually tip the connector up so the cable is vertical above the connector and heat the connector just enough to allow some applied solder to run down inside the connector but not so much that it would run/wick up inside the cable sheathing and make that area stiff. either way, when done the process, the shrink tube is brought down over the connector to cable joint and shrunk down. use the thick double walled tube because it contains a hot glue inside that will stick to the components and keep the area insulated and dry.
thats just me though, it's your project so do it like you want it. larger gauge wire is better than smaller, marginally sized, wire so if you decide to upgrade to a circuit with more draw, like add a component later, the wire will still handle it.
below is a link to a site that explains the different styles of crimps and when/where to use them. which are better and why etc. have you also tried an electrical supply outlet that wholesalers would use? normal wholesalers also sell regular stranded copper wire as well, not just the single strand "house" wire. maybe a little cheaper. automotive stereo install shops are also another source for cables, fuse blocks, etc. welding supply places sell cable off a roll usually and sometimes sell the terminals as well. it is a service for them so sometimes it is a little cheaper because they want the welder to also pick up some other supplies while there for the cable.
https://www.powerandcables.com/cable...s-of-crimping/
https://www.acklandsgrainger.com/en/...TYLE/p/GRO84-9
https://www.acklandsgrainger.com/en/...WG/p/WSW13H894