yes you can mix and match servos in an airplane....I would not mix a digital servo with an analog servo though.....A servo will be more powerful and quicker on 6 volt than 4.8.....
Hitech makes a 2.4 receiver I'm sure...I think it's called an extreme or something similiar to that.....have you thought about getting that pitts style muffler instead of that flex tube you were gonna use?