Some folks run 5 cell packs to get more speed and torque out of their servos, some will take it some wont, those w/o 6v specs wont. Also the thoery is on a 5 cell pack if a cell fails there will be enough voltage to run the reciever, if a 4 cell pack (4.8v) has a dead cell then you collect your plane with a dust buster.

Dual rates are a way of giving a lesser control throw with the flip of a switch making the plane less sensitive, many aerobatic planes use dual rates to land when over control would be a bad thing for example.