The transistor is just to turn the LEDs on and off.
Is the value of RXFB what's going to set the current that the NPN turns on/off?
No.
With the required current you must drop 0.6 V across that resistor so RXFB = 0.6 / the led current.
what do I need to look for in an NPN to work with this
One with enough gain to satisfy that equation, and sufficent to carry the required amount of current.