RE: basic esc/ battery and other questions
1. Yes, the ESC contains a Battery Eliminator Circuit (BEC) that steps down the battery voltage to the proper voltage for the receiver and servos. This is the standard practice for small to medium electric planes. The point is to save weight by avoiding the need to carry a second battery.
2. E-flight is always a compromise between performance and weight. A battery with more capacity will weigh more, perhaps so much more that the performance of the plane will actually decrease with the larger battery. The manufacturer has probably determined that the recommended battery is a good compromise between weight, thrust, speed, and flight time.
On the other hand, if you were to switch to lipo batteries, you could get greater capacity with no weight penalty. For the same capacity, lipos often weigh less than half as much as nickel-chemistry batteries. Lipos have pretty much revolutioned e-flight.
3. Using a larger ESC than you need is fine, assuming your airframe can tolerate the extra weight of the larger unit.
- Jeff