Using 6 volt battery won't hurt your receiver or servos......Now there are some retract servos that won't tolerate 6V...but you said you had electric eretracts so I assuming that you're using electric motors to retract the gear and not servos.....
So some basic electricity here....Voltage in series such as a flashlight were you install batteries positive to negative on top of each other the voltage is additive and the available current remains the same...In a parallel circuit such as two batteries to two different switches 1 switch to the battery port on your receiver and the other battery/switch to any unused channel on the reciever the voltage remains the same and the available current is additive....so in other words two 6V 1100 mAh batteries in parallel will give you 6V and 2200 mAh.....don't worry about the current the receiver will only get what it needs......
With electric retracts I'd go 6V with at least 1100 mAh as a minimum...1500 would be better and remember the current draw if you do touch and go's with the gear cycling....