There is no way to know what a safe charge rate is for a 2/3A battery. Charge rate has little to do with can size and everything to do with chemistry and construction. Without knowing the exact cells used, there will be no way to know maximum safe charge rate. My guess however is that the cells in the Electrifly pack are not designed for rapid charging mainly because rapid charge cells are more expensive, which would increase their cost, which reduces their market, in turn reduces thier profits, etc.

A lower voltage pack will reduce the power output of the motor. So yes, a 7.4V LiPo (regardless of its capacity) will deliver less power than a 9.6V pack. You could try a 3cell LiPo, which would be 11.1V, and would have more power. But if your system can't handle that voltage and subsequent current, smoke is going to come out of something. I'd stick with what the manufacture recommends until you know better.

I wouldn’t worry about matching the can size, i.e. AA size, as that doesn’t matter as long as the pack fits in the plane. Instead, match the voltage and capacity and you will be fine.