ok, lets explaine why 6v makes your servos faster...
the voltage is your potential.. the higher the voltage the more push you can have, with 6v you have more force on the current than at 4.8v therefore your servos can use that voltage to force more current through the motor and make it work faster and harder, however, a 4.8v pack will last longer than a 6v one because it's not pushing as much current ( so check your plane more often ) the wires and hardware in the plane will easily cope with the current 6v will produce under normal loads. 5 stalled servos might heat things up a bit tho.
get a peak charger that can do both ni-cad and hydrides, up to 9-10 cells is good too incase you go electric. something like the Hi-tec charger would be good. does your glow starter too.
as for discharge, dont discharge the flight packs at more than one amp. the cells can take it.. the wires cant