You can mix and match servos on a plane with no problems. The only place you'll want to make sure to use the same servos is when you have multiple servos on a control surface, such as 2 servos for ailerons, rudder, or elevators. You'll want the same type of servos in this case to ensure that both servos move the same with the same speed. Different brands of servos could be enough off each other to make a difference.

Ken