[bodywerks]

No need for a power expander at all.
The futaba rx can handle WAY more than 5 digi servos. It is not the receiver that is the limiting factor, it is the connectors on the servos and batteries. On a 50cc plane, a single battery input connector will work, although dual inputs is redundant and also capable of more amps, of course. I like Dick's setup recommendation, or if you want redundancy, go with dual A123 1100's and dual switches. If you are concerned about getting enough power to everything under the most extreme loads, a Fromeco DC-up, at a mere 1 ounce and like $35 is cheap insurance.
Personally, I'd go with a single A123 1100 with dual output leads, dual switches, and maybe a DC-up just to be safe. A 50cc plane only burns about 200-250milliamps per flight, so you could get a few flights easily and then field charge in a jiff.