Yes, a JR/Spektrum (or actually just about any other servo will plug into your Futaba radio). Servos are pretty much interchangeable these days.
When plugging them in, the plugs are "keyed" -- one side has its two corners slightly chamfered to prevent you from plugging it in upside down.
If you DO happen to plug them in upside down, nothing will be hurt, they just won't work.
As a reference:
JR/Spektrum Orange wire == Futaba White wire
JR/Spektrum Red wire == Futaba Red wire
JR/Spektrum Brown wire == Futaba Black wire
Some specialized servos (those ultra-fast servos designed to go with high performance gyros) will not work if directly plugged into the RX as they expect the neutral position pulse width to be 760 usec wide instead of 1520 usec.
Futaba plugs have a small tab molded into them. To use Futaba servos with anyone else's receiver, you need to simply trim that tab off the plug.
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