1) Any 4.8V battery with a compatible plug will work with that RX
2) You don't need to do anything to those servos. Just plug them in and use them
3) That TX battery will meet all of your needs for a long time. The people complaining about it are ones who are experienced enough to get long flights and many of them per day. There is a fix for the DX6's to replace the TX voltage regulator with a more efficient switching regulator, but that voids the warranty on the TX.
As long as you charge the battery before you go out to fly, your inexperience will be the limiting factor on your flight times.