Am I limited to 50 amps at the distro, due to the CS6364 plug? What happens if all circuits are trying to pull, for example, 10 amps at a time (80amps)?
You are limited by the "weak link" in the system. You should have overcurrent protection to protect the weakest link.
Assuming that large enough conductors are in use, there should be 50A double-pole, common-trip overcurrent protection for the entire distro, and 20A single-pole overcurrent protection for each 120V branch circuit receptacle (unless the wiring is only rated for 15A, in which case you should have 15A overcurrent protection).
Really, there are two parameters that determine the current-carrying capacity of a conductor: voltage drop and temperature rise. Voltage drop is a function of current and resistance (which is determined by cross-sectional area and length). Temperature rise is determined by current and the ability of the conductor to dissipate heat into the environment. (With temperature rise there is also an increase in resistance which results in increased voltage drop, but I don't know how significant that is.)
As for temperature rise, the primary concern is of the ability of the insulation to withstand heat before damage (charring) or failure (melting or ignition) and the ability of the surrounding environment to withstand heat from the wires (for example, an overloaded cord could cause dry grass to ignite before the insulation fails). The conductor's own resistance also means that a severely overloaded cord can melt the conductors.
The conductor's environment also has a limiting factor in the current capacity, as the environment can prevent heat dissipation. For example, in a bundle of wires, wires that are in the center of the bundle will not be able to dissipate heat as readily as those outside of the bundle. An extension cord that is left partially on a reel may overheat at a current level below its normal rating. But a single, uninsulated conductor hung in free air (as between two power poles) can easily dissipate heat, so may be able to carry current at a level far above the rating you find in a handy chart.
There are many factors that affect a conductor's current-carrying capacity. If you tried to make up a handy chart that takes all of these factors into account, it would no longer be handy; it would be even more confusing than what we have now. So when you do read a chart either in a manufacturer's literature, an electrical codebook, or a how-to manual, you need to consider the conditions that the chart is written for. A healthy dose of common sense helps, too. Trying to fudge the numbers to get off cheap probably isn't going to be a good thing. The only harm in oversizing wires is to the budget.
P.S. -- There is another factor to consider: durability. A 12 gauge cord when new has a typical rating of 20A. As the cord is flexed, the metal gradually becomes more and more brittle due to "cold working". This leads to metal fatigue, which can result in breakage of strands within the conductors. That means that over time with heavy use the safe capacity of the cord can diminish. A 10-year old 12 gauge cord that has seen daily use might only be good for 10A! You'll notice this most near the ends of the cord where flexion is the greatest: during use you'll see that the cord is much warmer near the ends than in the middle. Whenever you need to replace the end on a cord, trim off around 6-12" (~0.1-0.3m) of the cord to get past the worn out part.
