Hi-tec/JR and Futaba servos are all interchangeable. Some makes rotate in a different direction but this is not a problem providing you have reversing switches or can flip the servo arm through 180 degrees.

I always match the transmitter and receiver (more accurately I always use Futaba transmitters and receivers - personal preference) and a variety of Futaba/JR/Hi-tec servos. Whatever you do make sure that the crystals manufacturer is the same as the component it is plugged into (i.e. Futaba crystal in Futaba Transmitter / Hi-tec crystal in Hi-tec receiver etc)

Batterys are compatible and switches are too. Not sure about the charger though.

I hope someone can help with the rest of your questions.