You can use virtually any servo with your JR receiver.
Futaba servos have a "J" connector. Cut off the little tab that makes the "J" and they plug right in.
Hitec servos either have "J" or "S" connectors. The "S" connector plugs right in, the "J" needs trimming.
Modern Airtronics servos have "Z" connectors which plug right in to your receiver. Old Airtronics were wired differently.

The above assumes you're looking at new stuff. If you're looking at ancient stuff there's no telling what kind of plugs you'll encounter or how they'll be wired.

You can use a mixture of servo brands in the same model. I had a non-BB Hitec on the throttle of my trainer and BB Futabas on the control surfaces.