1.) Most models do not require flaps for taking off. They add drag. Unless you are lifting weight (like for a candy drop) flaps would be unnecessary as most models are WAY overepowered compared to full scale aircraft.

2.) Some models require down elevator when flaps are applied to remain level . . . but occasionally you will find one that requires some up elevator. Best policy is to add a small amount of flap, high up but at low speed, to see how your model reacts. Then mix in elevator and additional flap as required. Adding flap to a model at speed that is very aerobatic can cause bad things to happen. I tore the gear off my Hot Stik whan I accidently caught the flap toggle when switching to low rates and she took a 90º nose dive from 20 feet at half throttle. (And she can take off and do loops at 1/4 throttle). I barely had enough time to level her out before it was dirt nap time.