A few thoughts on your questions
A few thoughts from things I've picked up through the years, having been "the electrical guy" for a number of layouts and now getting around to building my second moderately sized layout. As a disclaimer, what follows are my opinions and observations. There are dozens of ways to do this, but this is what works for me.
Booster districts require a lot of thinking about your operating plan and adding up how much current you expect it to draw. For example, a large yard with a bunch of sound-equipped engines just idling will take a fair amount of current. Think about how you intend to operate, and how many engines will likely be in a given section at any given time, and give yourself some margin. For me, that involved thinking about what trains would run during an operation session, how many motors each would have, what happens when the big ones have to meet, where power would congregate when it wasn't doing anything. I'm in N, but I usually figure on 0.4A per engine without sound and 0.7A with sound when running at maximum load and maximum volu. Idling I figure is 0.1-0.2A. Modern HO engines don't draw that much more.
On top of boosters, I'd strongly consider adding breakers like the Powershield series. My theory on breakers is that nobody should ever come to a halt because a train they can't see derailed. (Likewise, the mainline should not stop because the yard trim job split a switch.) It frustrates the operators, leads to a lot of "okay, who's shorting things out" being yelled around, and generally destroys the illusion we're trying to create in an operating sesssion. So, each station area on my layout is one breaker district. In addition, I try to put the mainline only halfway to the next station in that same district, but it's not always possible. Sometimes there's no intermediate signal, and therefore no good place to have a separation between breakers, and thus the district runs all the way to the next station. But I try.
My personal opinion is to stick to 5A boosters if you don't plan to use circuit breakers. Less power to melt and weld things with shorts inevitably happen. If you do use good breakers, then it's less of a concern.
Put the switch machines on a separate power booster if you can. No need to run them off a separate system, but with separate power, when somebody runs through a switch and shorts out the track booster, you'll still be able to throw the switch and fix the problem. If the switch loses power because the track loses power, that's just an irritant.
The 30ft limit is a suggestion, not a limit. If your wiring isn't hideous, you can usually safely go beyond this. If you have the choice of locating the booster centrally to the district that it's powering and stay within 30 feet each direction, by all means, do that. There's no point in taunting the laws of physics and Mr. Murphy if you don't need to. However, there are some times it just doesn't work out. By "not hideous", I mean that you should always run everything in balanced pairs (meaning everywhere there's a wire for rail A, there should also be a wire for railb) and keep both track power wires together (preferably as 2 conductor cable or twisted individual conductors). Keep your track power bundle away from your signal lines (cab bus, DCC booster bus, etc.) Make good solid joints everywhere, and never skimp on wire gauge. The main danger of these long runs is lots of inductance, which can cause killer voltage spikes (well, killer to your decoders, anyway...) Good balanced wiring practices minimize this. If you have access to an oscilloscope, you can verify that you're not getting inductive voltage spikes that will kill decoders and such. If not, just throw an easily-made RC snubber circuit at the end of any excessively long bus for safety.
There's no standard color code. The important thing is that you pick a color code and you adhere to it like a zealot. My under-construction personal layout uses blue and white for DCC power, because I use 24AWG network cross-connect wire for track feeders and I could get bulk blue and white dual conductor wire dirt cheap. My 24V accessory power bus is yellow and black, 8-12V power is red and black, and then there's a myriad of colored Cat5 cables for data (grey for ethernet, green for MRBus, purple for serial, orange for NCE cab bus, blue for DCC booster data bus, yada yada yada...). Also, I'd strongly suggest wire labels and keeping a basic schematic of the layout in a notebook, noting color, size, and label of each run. In a big bundle of DCC lines, they really help you quickly identify which wire feeds what, even though they're all the same colors.
Terminal blocks vs. tapping - personal choice. I personally have quite a few blocks for signaling purposes, and each of those is fed by a bus that is soldered directly to the track feeders. Those busses then come back, go through block detectors, and into a terminal block where they connect to the main power district feeder line. That makes it possible to easily isolate any section by just pulling it off at the block detector, but I don't have a thousand terminal bars everywhere. If you're starting from scratch and careful, this works. However, on other more electrically "troubled" layouts that I've rewired, I use terminal bars everywhere so that I can find problems without cutting wires. It's a lifesaver.
As far as power wire and where to get it... I prefer stranded for everything just because it's easier to work with, so I skip the usual suggestions about romex and such. If you can bear a bit of experimentation on vendors and products, actually audio wire from places like eBay can be a tremendous bargain. Just be aware of two things: much of it is CCA (copper clad aluminum), which has only about 70% the conductivity of pure copper, and some of it is woefully undersized for the advertised wire gauge. The conductivity issue can be solved by just running about 2AWG bigger (so 12 rather than 14), and the "woefully undersized" bit is solved by avoiding certain vendors. Otherwise, the stuff is wonderful to work with - flexible insulation, finely stranded, copper clad so it oxidizes and solders just like copper, and unbelievably cheap. I picked up 500ft of dual conductor 12AWG CCA for a bit over 70 bucks about a year ago. It really is 12 AWG, and I measured the resistance when it came in, and it's just about that of 14 AWG pure copper. That's my main bus line. The local sub-busses are 16AWG pure copper that I also picked up off eBay, because I didn't want to deal with such large conductors locally.
I realize I've rambled on, but hopefully that helps.
Nathan H
