The problem with the 7C is that the knob will stay active unless you're using channel 6 for the second aileron servo by turning on the flaperon function. Your best bet is to use the flaperon function for your two aileron servos and then set channel 5 to the switch you use to activate the rudder retract function. Also, if you're not using a retract servo, which turns 180 degrees, you may not have enough travel with a standard servo, but that's all depending upon how the rudder retract function's set up.
This eliminates a mix. Since you have regular flaps on channel 7, you don't activate the Flap Trim function, which is used to make the ailerons move as flaps in addition to their regular function, and it controlled by the knob. When you're not using the ailerons that way, the knob is inactive.