RE: Electric power question
The biggest problem with putting motors in series is that the cell count oftens exceeds the ESC limit for use with BEC. This means more weight and cost by using either an Rx. battery pack or an Ultimate BEC device.
The 16 cells of 1000mAh HECELL or KAN1050 NiMH that deliver 10amps will rougly weigh a similar amount to 8-cells of 1900SCR or 1950mAh FAUP NiMH that deliver 20amps. Overall, the lower cell count is cheaper and will weigh less with progressive technologies like Lithium Polymer.
So, in summary, keep it simple! Use the motors in parallel with a single ESC on less cells.
Good luck!