Joe,
I'm sorry that you missed Nick's earlier threads on his new layout. If you had seen the posts, you would know that this layout is a temporary one and total space available is very limited, i.e. future expansion with this one is not likely.
Your point of not knowing a proper size needed for the bus because of future expansion is exactly why you want to go with a larger wire in the beginning. Most cannot understand the relationships between wire size, distance, and the total current that the wire will handle. That is why most recommendations are made in general rather than specifics. It is better to be too large than too little.
My layout has grown from a half-garage to a full garage size, but fortunately I began using 12awg for my bus system with many parallel branches. I don't have a booster and I don't experience any significant voltage loss and the "coin test" works fine.
I'm not sure I understand what you are saying about your layout bus size. If you are saying it is OK as is, I agree that there is nothing wrong with you using 16 gauge for 60 feet, but better if you divide the busing into parallel branches of short lengths. You are only looking at 4.26 ohms of resistance per 1000 feet for 16awg or 2.68 ohms/1000 for 14awg.
Your point of soldering/connection difficulties is well taken. When splicing bus wires of a large gauge, it is electrically correct to use twist connectors (top hats) or even split-bolt couplers to
connect to each other.
However, when soldering smaller parallel track feeders to the bus, I simply pull back the insulation of the bus, clean, wrap wire and solder. I prefer a solid bus wire (12awg) simply because it is easier to form and also to solder to, but this is a personal preference.
I am curious as to what you meant about the wire separation issues. The DCC signal is so relatively slow, a need for wire twist (e.g. spec. Cat 5) or parallel runs for prevention of signal concellation/crosstalk has not been a problem that I am aware of. More of concern would be avoiding noise sources.