It's a science, but it doesn't have to be an exact science.

You know, I think if you ask any of the glow converts here, that not a single solitary one has ever worried about RPMs or horsepower on a glow engine when powering an airplane. They choose their power system one of two ways:

1. Use what's recommended on the box.
2. Use the same engine as what's commonly used on planes of similar performance, size and weight.

Regarding your batteries, size doesn't just affect flying time. Every LiPoly has a hard limit as to how much current it can produce without sustaining signifigant, permanent damage. It's called the "C rating," and is expressed in a multiple of the battery's capacity. The majority of LiPolys on the market are rated at 10C, meaning that they can handle 10 times their capacity in current. For your TP1320 and TP1500mAh packs, that means 13.2 Amps and 15 Amps respectively. Try to draw 19 Amps from either of them, and you'll fry them. You can use them if you gang two packs in parallel to share the load.

It's the motor/gearbox/prop combination that determines how many Amps will be drawn. Everything else in the system is passive. The ESC only lets current through; it doesn't limit or regulate it, so you can easily draw 20 Amps through a 10 Amp ESC and fry it, for example. Same thing goes for the battery...