Can I have my ac accessory power go through a spst switch then to a terminal block and then branch to various led and incandescent lights if all my leds have appropriate diodes or rectifiers.
What rating of switch should I look for and what size wire from terminal block to led leads? Does wire going to incand. bulbs need to be bigger? Totally uncomfortable with wiring so I don't even know if this is appropriate.
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You don't need #18 wire for that mix of stuff, #22 would be plenty! The only precaution I'd take is to add a 3A fast-acting fuse at the transformer to protect the wiring in case of a short. The load you're describing is less than an amp, #18 is overkill unless you have it or are planning a significant expansion.
If you ran 100 feet of #22, and you had a 1A load on it, it would drop about 3 volts. That's considering a total of 200 feet of wire run, as you have the return path. Less load, less voltage drop. Obviously, if the run from the transformer is shorter, as I'm guessing it is, you'd have even less voltage drop in the wiring.
#22 is good for 5 amps enclosed, and you don't appear to be anywhere close to that. The bonus of smaller wire is it's easier to work with and conceal.
Here's an Ampacity chart for common wire sizes.
I only point out the #22 if he wants smaller wire to hide it. As you say, if size isn't a major issue, I'd probably go with #16 lamp cord, it's pretty cheap at most outlets.
I think the Miller signs would be hard pressed to use 1/3 of an amp, they are very low current, and they use a switching supply. I measured the GE and Western Auto signs, the peak for the larger GE was around 130ma with everything lit. They both require 4.5 volts DC to their boards. Even a linear supply would be only a max of around 150ma for the larger sign.
In any case, I think the wire size is pretty flexible. 
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